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<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2016</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.

    </legalnotice>

    <title>lists</title>
    <prepared>Robert Virding</prepared>
    <docno>1</docno>
    <date>1996-09-28</date>
    <rev>A</rev>
  </header>
  <module>lists</module>
  <modulesummary>List processing functions.</modulesummary>
  <description>
    <p>This module contains functions for list processing.</p>

    <p>Unless otherwise stated, all functions assume that position
      numbering starts at 1. That is, the first element of a list is at
      position 1.</p>

    <p>Two terms <c>T1</c> and <c>T2</c> compare equal if
      <c>T1&nbsp;==&nbsp;T2</c> evaluates to <c>true</c>. They match
      if <c>T1&nbsp;=:=&nbsp;T2</c> evaluates to <c>true</c>.</p>

    <p>Whenever an <marker
      id="ordering_function"></marker><em>ordering function</em>
      <c>F</c> is expected as argument, it is assumed that the
      following properties hold of <c>F</c> for all x, y, and z:</p>

    <list type="bulleted">
      <item><p>If x <c>F</c> y and y <c>F</c> x, then x = y (<c>F</c>
        is antisymmetric).</p>
      </item>
      <item><p>If x <c>F</c> y and y <c>F</c> z, then x <c>F</c> z
        (<c>F</c> is transitive).</p>
      </item>
      <item><p>x <c>F</c> y or y <c>F</c> x (<c>F</c> is total).</p>
      </item>
    </list>

    <p>An example of a typical ordering function is less than or equal
      to: <c>=&lt;/2</c>.</p>
  </description>

  <funcs>
    <func>
      <name name="all" arity="2"/>
      <fsummary>Return <c>true</c> if all elements in a list satisfy
        <c>Pred</c>.</fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>Pred</anno>(<anno>Elem</anno>)</c>
          returns <c>true</c> for all elements <c><anno>Elem</anno></c> in
          <c><anno>List</anno></c>, otherwise <c>false</c>.</p>
      </desc>
    </func>

    <func>
      <name name="any" arity="2"/>
      <fsummary>Return <c>true</c> if any of the elements in a list
        satisfies <c>Pred</c>.</fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>Pred</anno>(<anno>Elem</anno>)</c>
          returns <c>true</c> for at least one element <c><anno>Elem</anno></c>
          in <c><anno>List</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="append" arity="1"/>
      <fsummary>Append a list of lists.</fsummary>
      <desc>
        <p>Returns a list in which all the sublists of
          <c><anno>ListOfLists</anno></c> have been appended.</p>
        <p><em>Example:</em></p>
        <pre>
> <input>lists:append([[1, 2, 3], [a, b], [4, 5, 6]]).</input>
[1,2,3,a,b,4,5,6]</pre>
      </desc>
    </func>

    <func>
      <name name="append" arity="2"/>
      <fsummary>Append two lists.</fsummary>
      <desc>
        <p>Returns a new list <c><anno>List3</anno></c>, which is made from
          the elements of <c><anno>List1</anno></c> followed by the elements of
          <c><anno>List2</anno></c>.</p>
        <p><em>Example:</em></p>
        <pre>
> <input>lists:append("abc", "def").</input>
"abcdef"</pre>
        <p><c>lists:append(A, B)</c> is equivalent to <c>A ++ B</c>.</p>
      </desc>
    </func>

    <func>
      <name name="concat" arity="1"/>
      <fsummary>Concatenate a list of atoms.</fsummary>
      <desc>
        <p>Concatenates the text representation of the elements of
          <c><anno>Things</anno></c>. The elements of <c><anno>Things</anno></c>
          can be atoms, integers, floats, or strings.</p>
        <p><em>Example:</em></p>
        <pre>
> <input>lists:concat([doc, '/', file, '.', 3]).</input>
"doc/file.3"</pre>
      </desc>
    </func>

    <func>
      <name name="delete" arity="2"/>
      <fsummary>Delete an element from a list.</fsummary>
      <desc>
        <p>Returns a copy of <c><anno>List1</anno></c> where the first element
          matching <c><anno>Elem</anno></c> is deleted, if there is such an
          element.</p>
      </desc>
    </func>

    <func>
      <name name="droplast" arity="1"/>
      <fsummary>Drop the last element of a list.</fsummary>
      <desc>
        <p>Drops the last element of a <c><anno>List</anno></c>. The list is to
           be non-empty, otherwise the function crashes with a
           <c>function_clause</c>.</p>
      </desc>
    </func>

    <func>
      <name name="dropwhile" arity="2"/>
      <fsummary>Drop elements from a list while a predicate is <c>true</c>.
      </fsummary>
      <desc>
        <p>Drops elements <c><anno>Elem</anno></c> from
          <c><anno>List1</anno></c> while
          <c><anno>Pred</anno>(<anno>Elem</anno>)</c> returns <c>true</c> and
          returns the remaining list.</p>
      </desc>
    </func>

    <func>
      <name name="duplicate" arity="2"/>
      <fsummary>Make <c>N</c> copies of element.</fsummary>
      <desc>
        <p>Returns a list containing <c><anno>N</anno></c> copies of term
          <c><anno>Elem</anno></c>.</p>
        <p><em>Example:</em></p>
        <pre>
> <input>lists:duplicate(5, xx).</input>
[xx,xx,xx,xx,xx]</pre>
      </desc>
    </func>

    <func>
      <name name="filter" arity="2"/>
      <fsummary>Select elements that satisfy a predicate.</fsummary>
      <desc>
        <p><c><anno>List2</anno></c> is a list of all elements
          <c><anno>Elem</anno></c> in <c><anno>List1</anno></c> for which
          <c><anno>Pred</anno>(<anno>Elem</anno>)</c> returns <c>true</c>.</p>
      </desc>
    </func>

    <func>
      <name name="filtermap" arity="2"/>
      <fsummary>Filter and map elements that satisfy a function.</fsummary>
      <desc>
        <p>Calls <c><anno>Fun</anno>(<anno>Elem</anno>)</c> on successive
          elements <c>Elem</c> of <c><anno>List1</anno></c>.
          <c><anno>Fun</anno>/2</c> must return either a Boolean or a tuple
          <c>{true, <anno>Value</anno>}</c>. The function returns the list of
          elements for which <c><anno>Fun</anno></c> returns a new value, where
          a value of <c>true</c> is synonymous with
          <c>{true, <anno>Elem</anno>}</c>.</p>
        <p>That is, <c>filtermap</c> behaves as if it had been defined as
          follows:</p>
        <code type="none">
filtermap(Fun, List1) ->
    lists:foldr(fun(Elem, Acc) ->
                       case Fun(Elem) of
                           false -> Acc;
                           true -> [Elem|Acc];
                           {true,Value} -> [Value|Acc]
                       end
                end, [], List1).</code>
        <p><em>Example:</em></p>
        <pre>
> <input>lists:filtermap(fun(X) -> case X rem 2 of 0 -> {true, X div 2}; _ -> false end end, [1,2,3,4,5]).</input>
[1,2]</pre>
      </desc>
    </func>

    <func>
      <name name="flatlength" arity="1"/>
      <fsummary>Length of flattened deep list.</fsummary>
      <desc>
        <p>Equivalent to <c>length(flatten(<anno>DeepList</anno>))</c>, but
          more efficient.</p>
      </desc>
    </func>

    <func>
      <name name="flatmap" arity="2"/>
      <fsummary>Map and flatten in one pass.</fsummary>
      <desc>
        <p>Takes a function from <c><anno>A</anno></c>s to lists of
          <c><anno>B</anno></c>s, and a list of <c><anno>A</anno></c>s
          (<c><anno>List1</anno></c>) and produces a list of
          <c><anno>B</anno></c>s by applying the function to every element in
          <c><anno>List1</anno></c> and appending the resulting lists.</p>
        <p>That is, <c>flatmap</c> behaves as if it had been defined as
          follows:</p>
        <code type="none">
flatmap(Fun, List1) ->
    append(map(Fun, List1)).</code>
        <p><em>Example:</em></p>
        <pre>
> <input>lists:flatmap(fun(X)->[X,X] end, [a,b,c]).</input>
[a,a,b,b,c,c]</pre>
      </desc>
    </func>

    <func>
      <name name="flatten" arity="1"/>
      <fsummary>Flatten a deep list.</fsummary>
      <desc>
        <p>Returns a flattened version of <c><anno>DeepList</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="flatten" arity="2"/>
      <fsummary>Flatten a deep list.</fsummary>
      <desc>
        <p>Returns a flattened version of <c><anno>DeepList</anno></c> with tail
          <c><anno>Tail</anno></c> appended.</p>
      </desc>
    </func>

    <func>
      <name name="foldl" arity="3"/>
      <fsummary>Fold a function over a list.</fsummary>
      <desc>
        <p>Calls <c><anno>Fun</anno>(<anno>Elem</anno>, <anno>AccIn</anno>)</c>
          on successive elements <c>A</c> of <c><anno>List</anno></c>, starting
          with <c><anno>AccIn</anno> == <anno>Acc0</anno></c>.
          <c><anno>Fun</anno>/2</c> must return a new accumulator, which is
          passed to the next call. The function returns the final value of
          the accumulator. <c><anno>Acc0</anno></c> is returned if the list is
          empty.</p>
        <p><em>Example:</em></p>
        <pre>
> <input>lists:foldl(fun(X, Sum) -> X + Sum end, 0, [1,2,3,4,5]).</input>
15
> <input>lists:foldl(fun(X, Prod) -> X * Prod end, 1, [1,2,3,4,5]).</input>
120</pre>
      </desc>
    </func>

    <func>
      <name name="foldr" arity="3"/>
      <fsummary>Fold a function over a list.</fsummary>
      <desc>
        <p>Like <seealso marker="#foldl/3"><c>foldl/3</c></seealso>, but the
          list is traversed from right to left.</p>
        <p><em>Example:</em></p>
        <pre>
> <input>P = fun(A, AccIn) -> io:format("~p ", [A]), AccIn end.</input>
#Fun&lt;erl_eval.12.2225172&gt;
> <input>lists:foldl(P, void, [1,2,3]).</input>
1 2 3 void
> <input>lists:foldr(P, void, [1,2,3]).</input>
3 2 1 void</pre>
        <p><c>foldl/3</c> is tail recursive and is usually preferred to
          <c>foldr/3</c>.</p>
      </desc>
    </func>

    <func>
      <name name="join" arity="2"/>
      <fsummary>Insert an element between elements in a list</fsummary>
      <desc>
        <p>Inserts <c><anno>Sep</anno></c> between each element in <c><anno>List1</anno></c>. Has no
          effect on the empty list and on a singleton list. For example:</p>
        <pre>
> <input>lists:join(x, [a,b,c]).</input>
[a,x,b,x,c]
> <input>lists:join(x, [a]).</input>
[a]
> <input>lists:join(x, []).</input>
[]</pre>
      </desc>
    </func>
    <func>
      <name name="foreach" arity="2"/>
      <fsummary>Apply a function to each element of a list.</fsummary>
      <desc>
        <p>Calls <c><anno>Fun</anno>(<anno>Elem</anno>)</c> for each element
          <c><anno>Elem</anno></c> in <c><anno>List</anno></c>. This function
          is used for its side effects and
          the evaluation order is defined to be the same as the order
          of the elements in the list.</p>
      </desc>
    </func>

    <func>
      <name name="keydelete" arity="3"/>
      <fsummary>Delete an element from a list of tuples.</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Returns a copy of <c><anno>TupleList1</anno></c> where the first
          occurrence of a tuple whose <c><anno>N</anno></c>th element compares
          equal to
          <c><anno>Key</anno></c> is deleted, if there is such a tuple.</p>
      </desc>
    </func>

    <func>
      <name name="keyfind" arity="3"/>
      <fsummary>Search for an element in a list of tuples.</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Searches the list of tuples <c><anno>TupleList</anno></c> for a
          tuple whose <c><anno>N</anno></c>th element compares equal to
          <c><anno>Key</anno></c>.
          Returns <c><anno>Tuple</anno></c> if such a tuple is found,
          otherwise <c>false</c>.</p>
      </desc>
    </func>

    <func>
      <name name="keymap" arity="3"/>
      <fsummary>Map a function over a list of tuples.</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Returns a list of tuples where, for each tuple in
          <c><anno>TupleList1</anno></c>, the <c><anno>N</anno></c>th element
          <c><anno>Term1</anno></c> of the tuple
          has been replaced with the result of calling
          <c><anno>Fun</anno>(<anno>Term1</anno>)</c>.</p>
        <p><em>Examples:</em></p>
        <pre>
> <input>Fun = fun(Atom) -> atom_to_list(Atom) end.</input>
#Fun&lt;erl_eval.6.10732646&gt;
2> <input>lists:keymap(Fun, 2, [{name,jane,22},{name,lizzie,20},{name,lydia,15}]).</input>
[{name,"jane",22},{name,"lizzie",20},{name,"lydia",15}]</pre>
      </desc>
    </func>

    <func>
      <name name="keymember" arity="3"/>
      <fsummary>Test for membership of a list of tuples.</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Returns <c>true</c> if there is a tuple in
          <c><anno>TupleList</anno></c> whose <c><anno>N</anno></c>th element
          compares equal to <c><anno>Key</anno></c>, otherwise <c>false</c>.</p>
      </desc>
    </func>

    <func>
      <name name="keymerge" arity="3"/>
      <fsummary>Merge two key-sorted lists of tuples.</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Returns the sorted list formed by merging
          <c><anno>TupleList1</anno></c> and <c><anno>TupleList2</anno></c>.
          The merge is performed on the <c><anno>N</anno></c>th element of each
          tuple. Both <c><anno>TupleList1</anno></c> and
          <c><anno>TupleList2</anno></c> must be key-sorted before evaluating
          this function. When two tuples compare equal, the tuple from
          <c><anno>TupleList1</anno></c> is picked before the tuple from
          <c><anno>TupleList2</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="keyreplace" arity="4"/>
      <fsummary>Replace an element in a list of tuples.</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Returns a copy of <c><anno>TupleList1</anno></c> where the first
          occurrence of a <c>T</c> tuple whose <c><anno>N</anno></c>th element
          compares equal to <c><anno>Key</anno></c> is replaced with
          <c><anno>NewTuple</anno></c>, if there is such a tuple <c>T</c>.</p>
      </desc>
    </func>

    <func>
      <name name="keysearch" arity="3"/>
      <fsummary>Search for an element in a list of tuples.</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Searches the list of tuples <c><anno>TupleList</anno></c> for a
          tuple whose <c><anno>N</anno></c>th element compares equal to
          <c><anno>Key</anno></c>.
          Returns <c>{value, <anno>Tuple</anno>}</c> if such a tuple is found,
          otherwise <c>false</c>.</p>
      <note>
        <p>This function is retained for backward compatibility. Function
          <seealso marker="#keyfind/3"><c>keyfind/3</c></seealso>
          is usually more convenient.</p>
      </note>
      </desc>
    </func>

    <func>
      <name name="keysort" arity="2"/>
      <fsummary>Sort a list of tuples.</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Returns a list containing the sorted elements of list
          <c><anno>TupleList1</anno></c>. Sorting is performed on the
          <c><anno>N</anno></c>th element of the tuples. The sort is stable.</p>
      </desc>
    </func>

    <func>
      <name name="keystore" arity="4"/>
      <fsummary>Store an element in a list of tuples.</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Returns a copy of <c><anno>TupleList1</anno></c> where the first
          occurrence of a tuple <c>T</c> whose <c><anno>N</anno></c>th element
          compares equal to <c><anno>Key</anno></c> is replaced with
          <c><anno>NewTuple</anno></c>, if there is such a tuple <c>T</c>.
          If there is no such tuple <c>T</c>, a copy of
          <c><anno>TupleList1</anno></c> where
          [<c><anno>NewTuple</anno></c>] has been appended to the end is
          returned.</p>
      </desc>
    </func>

    <func>
      <name name="keytake" arity="3"/>
      <fsummary>Extract an element from a list of tuples.</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Searches the list of tuples <c><anno>TupleList1</anno></c> for a
          tuple whose <c><anno>N</anno></c>th element compares equal to
          <c><anno>Key</anno></c>. Returns <c>{value, <anno>Tuple</anno>,
          <anno>TupleList2</anno>}</c> if such a tuple is found, otherwise
          <c>false</c>. <c><anno>TupleList2</anno></c> is a copy
          of <c><anno>TupleList1</anno></c> where the first occurrence of
          <c><anno>Tuple</anno></c> has been removed.</p>
      </desc>
    </func>

    <func>
      <name name="last" arity="1"/>
      <fsummary>Return last element in a list.</fsummary>
      <desc>
        <p>Returns the last element in <c><anno>List</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="map" arity="2"/>
      <fsummary>Map a function over a list.</fsummary>
      <desc>
        <p>Takes a function from <c><anno>A</anno></c>s to
          <c><anno>B</anno></c>s, and a list of <c><anno>A</anno></c>s and
          produces a list of <c><anno>B</anno></c>s by applying
          the function to every element in the list. This function is
          used to obtain the return values. The evaluation order depends on
          the implementation.</p>
      </desc>
    </func>

    <func>
      <name name="mapfoldl" arity="3"/>
      <fsummary>Map and fold in one pass.</fsummary>
      <desc>
        <p>Combines the operations of
          <seealso marker="#map/2"><c>map/2</c></seealso> and 
          <seealso marker="#foldl/3"><c>foldl/3</c></seealso> into one pass.</p>
        <p><em>Example:</em></p>
        <p>Summing the elements in a list and double them at the same time:</p>
        <pre>
> <input>lists:mapfoldl(fun(X, Sum) -> {2*X, X+Sum} end,</input>
<input>0, [1,2,3,4,5]).</input>
{[2,4,6,8,10],15}</pre>
      </desc>
    </func>

    <func>
      <name name="mapfoldr" arity="3"/>
      <fsummary>Map and fold in one pass.</fsummary>
      <desc>
        <p>Combines the operations of
          <seealso marker="#map/2"><c>map/2</c></seealso> and 
          <seealso marker="#foldr/3"><c>foldr/3</c></seealso> into one pass.</p>
      </desc>
    </func>

    <func>
      <name name="max" arity="1"/>
      <fsummary>Return maximum element of a list.</fsummary>
      <desc>
        <p>Returns the first element of <c><anno>List</anno></c> that compares
          greater than or equal to all other elements of
          <c><anno>List</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="member" arity="2"/>
      <fsummary>Test for membership of a list.</fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>Elem</anno></c> matches some element
          of <c><anno>List</anno></c>, otherwise <c>false</c>.</p>
      </desc>
    </func>

    <func>
      <name name="merge" arity="1"/>
      <fsummary>Merge a list of sorted lists.</fsummary>
      <desc>
        <p>Returns the sorted list formed by merging all the sublists of
          <c><anno>ListOfLists</anno></c>. All sublists must be sorted before
          evaluating this function. When two elements compare equal,
          the element from the sublist with the lowest position in
          <c><anno>ListOfLists</anno></c> is picked before the other
          element.</p>
      </desc>
    </func>

    <func>
      <name name="merge" arity="2"/>
      <fsummary>Merge two sorted lists.</fsummary>
      <desc>
        <p>Returns the sorted list formed by merging <c><anno>List1</anno></c>
          and <c><anno>List2</anno></c>. Both <c><anno>List1</anno></c> and
          <c><anno>List2</anno></c> must be
          sorted before evaluating this function. When two elements
          compare equal, the element from <c><anno>List1</anno></c> is picked
          before the element from <c><anno>List2</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="merge" arity="3"/>
      <fsummary>Merge two sorted list.</fsummary>
      <desc>
        <p>Returns the sorted list formed by merging <c><anno>List1</anno></c>
          and <c><anno>List2</anno></c>. Both <c><anno>List1</anno></c> and
          <c><anno>List2</anno></c> must be sorted according to the <seealso
          marker="#ordering_function">ordering function</seealso>
          <c><anno>Fun</anno></c> before evaluating this function.
          <c><anno>Fun</anno>(<anno>A</anno>, <anno>B</anno>)</c> is to return
          <c>true</c> if <c><anno>A</anno></c> compares less
          than or equal to <c><anno>B</anno></c> in the ordering, otherwise
          <c>false</c>. When two elements compare equal, the element from
          <c><anno>List1</anno></c> is picked before the element from
          <c><anno>List2</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="merge3" arity="3"/>
      <fsummary>Merge three sorted lists.</fsummary>
      <desc>
        <p>Returns the sorted list formed by merging <c><anno>List1</anno></c>,
          <c><anno>List2</anno></c>, and <c><anno>List3</anno></c>. All of
          <c><anno>List1</anno></c>, <c><anno>List2</anno></c>, and
          <c><anno>List3</anno></c> must be sorted before evaluating this
          function. When two elements compare equal, the element from
          <c><anno>List1</anno></c>, if there is such an element,
          is picked before the other element, otherwise the element
          from <c><anno>List2</anno></c> is picked before the element from
          <c><anno>List3</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="min" arity="1"/>
      <fsummary>Return minimum element of a list.</fsummary>
      <desc>
        <p>Returns the first element of <c><anno>List</anno></c> that compares
          less than or equal to all other elements of
          <c><anno>List</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="nth" arity="2"/>
      <fsummary>Return the <c>N</c>th element of a list.</fsummary>
      <type_desc variable="N">1..length(<anno>List</anno>)</type_desc>
      <desc>
        <p>Returns the <c><anno>N</anno></c>th element of
          <c><anno>List</anno></c>.</p>
        <p><em>Example:</em></p>
        <pre>
> <input>lists:nth(3, [a, b, c, d, e]).</input>
c</pre>
      </desc>
    </func>

    <func>
      <name name="nthtail" arity="2"/>
      <fsummary>Return the <c>N</c>th tail of a list.</fsummary>
      <type_desc variable="N">0..length(<anno>List</anno>)</type_desc>
      <desc>
        <p>Returns the <c><anno>N</anno></c>th tail of <c><anno>List</anno></c>,
          that is, the sublist of <c><anno>List</anno></c> starting at
          <c><anno>N</anno>+1</c> and continuing up to the end of the list.</p>
        <p><em>Example</em></p>
        <pre>
> <input>lists:nthtail(3, [a, b, c, d, e]).</input>
[d,e]
> <input>tl(tl(tl([a, b, c, d, e]))).</input>
[d,e]
> <input>lists:nthtail(0, [a, b, c, d, e]).</input>
[a,b,c,d,e]
> <input>lists:nthtail(5, [a, b, c, d, e]).</input>
[]</pre>
      </desc>
    </func>

    <func>
      <name name="partition" arity="2"/>
      <fsummary>Partition a list into two lists based on a predicate.</fsummary>
      <desc>
        <p>Partitions <c><anno>List</anno></c> into two lists, where the first
          list contains all elements for which
          <c><anno>Pred</anno>(<anno>Elem</anno>)</c> returns <c>true</c>,
          and the second list contains all elements for which
          <c><anno>Pred</anno>(<anno>Elem</anno>)</c> returns <c>false</c>.</p>
        <p><em>Examples:</em></p>
        <pre>
> <input>lists:partition(fun(A) -> A rem 2 == 1 end, [1,2,3,4,5,6,7]).</input>
{[1,3,5,7],[2,4,6]}
> <input>lists:partition(fun(A) -> is_atom(A) end, [a,b,1,c,d,2,3,4,e]).</input>
{[a,b,c,d,e],[1,2,3,4]}</pre>
        <p>For a different way to partition a list, see
          <seealso marker="#splitwith/2"><c>splitwith/2</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="prefix" arity="2"/>
      <fsummary>Test for list prefix.</fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>List1</anno></c> is a prefix of
          <c><anno>List2</anno></c>, otherwise <c>false</c>.</p>
      </desc>
    </func>

    <func>
      <name name="reverse" arity="1"/>
      <fsummary>Reverse a list.</fsummary>
      <desc>
        <p>Returns a list with the elements in <c><anno>List1</anno></c>
          in reverse order.</p>
      </desc>
    </func>

    <func>
      <name name="reverse" arity="2"/>
      <fsummary>Reverse a list appending a tail.</fsummary>
      <desc>
        <p>Returns a list with the elements in <c><anno>List1</anno></c>
          in reverse order, with tail <c><anno>Tail</anno></c> appended.</p>
        <p><em>Example:</em></p>
        <pre>
> <input>lists:reverse([1, 2, 3, 4], [a, b, c]).</input>
[4,3,2,1,a,b,c]</pre>
      </desc>
    </func>

    <func>
      <name name="seq" arity="2"/>
      <name name="seq" arity="3"/>
      <fsummary>Generate a sequence of integers.</fsummary>
      <desc>
        <p>Returns a sequence of integers that starts with
          <c><anno>From</anno></c> and contains the successive results of
          adding <c><anno>Incr</anno></c> to the previous element, until
          <c><anno>To</anno></c> is reached or passed (in the latter case,
          <c><anno>To</anno></c> is not an element of
          the sequence). <c><anno>Incr</anno></c> defaults to 1.</p>
        <p>Failures:</p>
        <list type="bulleted">
          <item>
            <p>If <c><anno>To</anno> &lt;
	      <anno>From</anno> - <anno>Incr</anno></c>
              and <c><anno>Incr</anno> &gt; 0</c>.</p>
          </item>
          <item>
            <p>If <c><anno>To</anno> &gt;
              <anno>From</anno> - <anno>Incr</anno></c> and
              <c><anno>Incr</anno> &lt; 0</c>.</p>
          </item>
          <item>
            <p>If <c><anno>Incr</anno> =:= 0</c> and
              <c><anno>From</anno> =/= <anno>To</anno></c>.</p>
          </item>
        </list>
        <p>The following equalities hold for all sequences:</p>
        <code type="none">
length(lists:seq(From, To)) =:= To - From + 1
length(lists:seq(From, To, Incr)) =:= (To - From + Incr) div Incr</code>
        <p><em>Examples:</em></p>
        <pre>
> <input>lists:seq(1, 10).</input>
[1,2,3,4,5,6,7,8,9,10]
> <input>lists:seq(1, 20, 3).</input>
[1,4,7,10,13,16,19]
> <input>lists:seq(1, 0, 1).</input>
[]
> <input>lists:seq(10, 6, 4).</input>
[]
> <input>lists:seq(1, 1, 0).</input>
[1]</pre>
      </desc>
    </func>

    <func>
      <name name="sort" arity="1"/>
      <fsummary>Sort a list.</fsummary>
      <desc>
        <p>Returns a list containing the sorted elements of
          <c><anno>List1</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="sort" arity="2"/>
      <fsummary>Sort a list.</fsummary>
      <desc>
        <p>Returns a list containing the sorted elements of
          <c><anno>List1</anno></c>, according to the <seealso
          marker="#ordering_function">ordering function</seealso>
          <c><anno>Fun</anno></c>. <c><anno>Fun</anno>(<anno>A</anno>,
          <anno>B</anno>)</c> is to return <c>true</c> if <c><anno>A</anno></c>
          compares less than or equal to <c><anno>B</anno></c> in the
          ordering, otherwise <c>false</c>.</p>
      </desc>
    </func>

    <func>
      <name name="split" arity="2"/>
      <fsummary>Split a list into two lists.</fsummary>
      <type_desc variable="N">0..length(<anno>List1</anno>)</type_desc>
      <desc>
        <p>Splits <c><anno>List1</anno></c> into <c><anno>List2</anno></c> and
          <c><anno>List3</anno></c>. <c><anno>List2</anno></c> contains the
          first <c><anno>N</anno></c> elements and <c><anno>List3</anno></c>
          the remaining elements (the <c><anno>N</anno></c>th tail).</p>
      </desc>
    </func>

    <func>
      <name name="splitwith" arity="2"/>
      <fsummary>Split a list into two lists based on a predicate.</fsummary>
      <desc>
        <p>Partitions <c><anno>List</anno></c> into two lists according to
          <c><anno>Pred</anno></c>. <c>splitwith/2</c> behaves as if it is
          defined as follows:</p>
        <code type="none">
splitwith(Pred, List) ->
    {takewhile(Pred, List), dropwhile(Pred, List)}.</code>
        <p><em>Examples:</em></p>
        <pre>
> <input>lists:splitwith(fun(A) -> A rem 2 == 1 end, [1,2,3,4,5,6,7]).</input>
{[1],[2,3,4,5,6,7]}
> <input>lists:splitwith(fun(A) -> is_atom(A) end, [a,b,1,c,d,2,3,4,e]).</input>
{[a,b],[1,c,d,2,3,4,e]}</pre>
        <p>For a different way to partition a list, see 
          <seealso marker="#partition/2"><c>partition/2</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="sublist" arity="2"/>
      <fsummary>Return a sublist of a certain length, starting at the first
        position.</fsummary>
      <desc>
        <p>Returns the sublist of <c><anno>List1</anno></c> starting at
          position 1 and with (maximum) <c><anno>Len</anno></c> elements. It is
          not an error for <c><anno>Len</anno></c> to exceed the length of the
          list, in that case the whole list is returned.</p>
      </desc>
    </func>

    <func>
      <name name="sublist" arity="3"/>
      <fsummary>Return a sublist starting at a specified position and with a
        specified number of elements.</fsummary>
      <type_desc variable="Start">1..(length(<anno>List1</anno>)+1)</type_desc>
      <desc>
        <p>Returns the sublist of <c><anno>List1</anno></c> starting at
          <c><anno>Start</anno></c> and with (maximum) <c><anno>Len</anno></c>
          elements. It is not an error for
          <c><anno>Start</anno>+<anno>Len</anno></c> to exceed the length of
          the list.</p>
        <p><em>Examples:</em></p>
        <pre>
> <input>lists:sublist([1,2,3,4], 2, 2).</input>
[2,3]
> <input>lists:sublist([1,2,3,4], 2, 5).</input>
[2,3,4]
> <input>lists:sublist([1,2,3,4], 5, 2).</input>
[]</pre>
      </desc>
    </func>

    <func>
      <name name="subtract" arity="2"/>
      <fsummary>Subtract the element in one list from another list.</fsummary>
      <desc>
        <p>Returns a new list <c><anno>List3</anno></c> that is a copy of
          <c><anno>List1</anno></c>, subjected to the following procedure:
          for each element in <c><anno>List2</anno></c>, its first occurrence
          in <c><anno>List1</anno></c> is deleted.</p>
        <p><em>Example:</em></p>
        <pre>
> <input>lists:subtract("123212", "212").</input>
"312".</pre>
        <p><c>lists:subtract(A, B)</c> is equivalent to <c>A -- B</c>.</p>
        <warning>
          <p>The complexity of <c>lists:subtract(A, B)</c> is proportional to
            <c>length(A)*length(B)</c>, meaning that it is very slow if both
            <c>A</c> and <c>B</c> are long lists. (If both lists are long, it
            is a much better choice to use ordered lists and
            <seealso marker="ordsets#subtract/2">
            <c>ordsets:subtract/2</c></seealso>.</p>
        </warning>
      </desc>
    </func>

    <func>
      <name name="suffix" arity="2"/>
      <fsummary>Test for list suffix.</fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>List1</anno></c> is a suffix of
          <c><anno>List2</anno></c>, otherwise <c>false</c>.</p>
      </desc>
    </func>

    <func>
      <name name="sum" arity="1"/>
      <fsummary>Return the sum of elements in a list.</fsummary>
      <desc>
        <p>Returns the sum of the elements in <c><anno>List</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="takewhile" arity="2"/>
      <fsummary>Take elements from a list while a predicate is <c>true</c>.
      </fsummary>
      <desc>
        <p>Takes elements <c><anno>Elem</anno></c> from
          <c><anno>List1</anno></c> while
          <c><anno>Pred</anno>(<anno>Elem</anno>)</c> returns <c>true</c>, that
          is, the function returns the longest prefix of the list for which
          all elements satisfy the predicate.</p>
      </desc>
    </func>

    <func>
      <name name="ukeymerge" arity="3"/>
      <fsummary>Merge two key-sorted lists of tuples, removing duplicates.
      </fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Returns the sorted list formed by merging
          <c><anno>TupleList1</anno></c> and
          <c><anno>TupleList2</anno></c>. The merge is performed on the
          <c><anno>N</anno></c>th element of each tuple. Both
          <c><anno>TupleList1</anno></c> and <c><anno>TupleList2</anno></c>
          must be key-sorted without duplicates before evaluating this function.
          When two tuples compare equal, the tuple from
          <c><anno>TupleList1</anno></c> is picked and the
          one from <c><anno>TupleList2</anno></c> is deleted.</p>
      </desc>
    </func>

    <func>
      <name name="ukeysort" arity="2"/>
      <fsummary>Sort a list of tuples, removing duplicates.</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Returns a list containing the sorted elements of list
          <c><anno>TupleList1</anno></c> where all except the first tuple of
          the tuples comparing equal have been deleted. Sorting is
          performed on the <c><anno>N</anno></c>th element of the tuples.</p>
      </desc>
    </func>

    <func>
      <name name="umerge" arity="1"/>
      <fsummary>Merge a list of sorted lists, removing duplicates.</fsummary>
      <desc>
        <p>Returns the sorted list formed by merging all the sublists
          of <c><anno>ListOfLists</anno></c>. All sublists must be sorted and
          contain no duplicates before evaluating this function.
          When two elements compare equal, the element from the sublist
          with the lowest position in <c><anno>ListOfLists</anno></c> is
          picked and the other is deleted.</p>
      </desc>
    </func>

    <func>
      <name name="umerge" arity="2"/>
      <fsummary>Merge two sorted lists, removing duplicates.</fsummary>
      <desc>
        <p>Returns the sorted list formed by merging <c><anno>List1</anno></c>
          and <c><anno>List2</anno></c>. Both <c><anno>List1</anno></c> and
          <c><anno>List2</anno></c> must be
          sorted and contain no duplicates before evaluating this
          function. When two elements compare equal, the element from
          <c><anno>List1</anno></c> is picked and the one from
          <c><anno>List2</anno></c> is deleted.</p>
      </desc>
    </func>

    <func>
      <name name="umerge" arity="3"/>
      <fsummary>Merge two sorted lists, removing duplicates.</fsummary>
      <desc>
        <p>Returns the sorted list formed by merging <c><anno>List1</anno></c>
          and <c><anno>List2</anno></c>. Both <c><anno>List1</anno></c> and
          <c><anno>List2</anno></c> must be sorted according to the <seealso
          marker="#ordering_function">ordering function</seealso>
          <c>Fun</c> and contain no duplicates before evaluating this function.
          <c><anno>Fun</anno>(<anno>A</anno>, <anno>B</anno>)</c> is to return
          <c>true</c> if <c><anno>A</anno></c> compares less than or equal to
          <c><anno>B</anno></c> in the ordering, otherwise <c>false</c>. When
          two elements compare equal, the element from <c><anno>List1</anno></c>
          is picked and the one from <c><anno>List2</anno></c> is deleted.</p>
      </desc>
    </func>

    <func>
      <name name="umerge3" arity="3"/>
      <fsummary>Merge three sorted lists, removing duplicates.</fsummary>
      <desc>
        <p>Returns the sorted list formed by merging <c><anno>List1</anno></c>,
          <c><anno>List2</anno></c>, and <c><anno>List3</anno></c>. All of
          <c><anno>List1</anno></c>, <c><anno>List2</anno></c>, and
          <c><anno>List3</anno></c> must be sorted and contain no
          duplicates before evaluating this function. When two
          elements compare equal, the element from <c><anno>List1</anno></c> is
          picked if there is such an element, otherwise the element from
          <c><anno>List2</anno></c> is picked, and the other is deleted.</p>
      </desc>
    </func>

    <func>
      <name name="unzip" arity="1"/>
      <fsummary>Unzip a list of two-tuples into two lists.</fsummary>
      <desc>
        <p>"Unzips" a list of two-tuples into two lists, where the first
          list contains the first element of each tuple, and the second
          list contains the second element of each tuple.</p>
      </desc>
    </func>

    <func>
      <name name="unzip3" arity="1"/>
      <fsummary>Unzip a list of three-tuples into three lists.</fsummary>
      <desc>
        <p>"Unzips" a list of three-tuples into three lists, where
          the first list contains the first element of each tuple,
          the second list contains the second element of each tuple, and
          the third list contains the third element of each tuple.</p>
      </desc>
    </func>

    <func>
      <name name="usort" arity="1"/>
      <fsummary>Sort a list, removing duplicates.</fsummary>
      <desc>
        <p>Returns a list containing the sorted elements of
          <c><anno>List1</anno></c> where all except the first element of the
          elements comparing equal have been deleted.</p>
      </desc>
    </func>

    <func>
      <name name="usort" arity="2"/>
      <fsummary>Sort a list, removing duplicates.</fsummary>
      <desc>
        <p>Returns a list containing the sorted elements of
          <c><anno>List1</anno></c> where all except the first element of the
          elements comparing equal according to the <seealso
          marker="#ordering_function">ordering function</seealso>
          <c><anno>Fun</anno></c> have been deleted.
          <c><anno>Fun</anno>(A, B)</c> is to return
          <c>true</c> if <c>A</c> compares less than or equal to
          <c>B</c> in the ordering, otherwise <c>false</c>.</p>
      </desc>
    </func>

    <func>
      <name name="zip" arity="2"/>
      <fsummary>Zip two lists into a list of two-tuples.</fsummary>
      <desc>
        <p>"Zips" two lists of equal length into one list of two-tuples,
          where the first element of each tuple is taken from the first
          list and the second element is taken from the corresponding
          element in the second list.</p>
      </desc>
    </func>

    <func>
      <name name="zip3" arity="3"/>
      <fsummary>Zip three lists into a list of three-tuples.</fsummary>
      <desc>
        <p>"Zips" three lists of equal length into one list of
          three-tuples, where the first element of each tuple is taken
          from the first list, the second element is taken from
          the corresponding element in the second list, and the third
          element is taken from the corresponding element in the third list.</p>
      </desc>
    </func>

    <func>
      <name name="zipwith" arity="3"/>
      <fsummary>Zip two lists into one list according to a fun.</fsummary>
      <desc>
        <p>Combines the elements of two lists of equal length into one list.
          For each pair <c><anno>X</anno>, <anno>Y</anno></c> of list elements
          from the two lists, the element in the result list is
          <c><anno>Combine</anno>(<anno>X</anno>, <anno>Y</anno>)</c>.</p>
        <p><c>zipwith(fun(X, Y) -> {X,Y} end, List1, List2)</c> is
          equivalent to <c>zip(List1, List2)</c>.</p>
        <p><em>Example:</em></p>
        <pre>
> <input>lists:zipwith(fun(X, Y) -> X+Y end, [1,2,3], [4,5,6]).</input>
[5,7,9]</pre>
      </desc>
    </func>

    <func>
      <name name="zipwith3" arity="4"/>
      <fsummary>Zip three lists into one list according to a fun.</fsummary>
      <desc>
        <p>Combines the elements of three lists of equal length into one
          list. For each triple <c><anno>X</anno>, <anno>Y</anno>,
          <anno>Z</anno></c> of list elements from the three lists, the element
          in the result list is <c><anno>Combine</anno>(<anno>X</anno>,
          <anno>Y</anno>, <anno>Z</anno>)</c>.</p>
        <p><c>zipwith3(fun(X, Y, Z) -> {X,Y,Z} end, List1, List2, List3)</c> is
          equivalent to <c>zip3(List1, List2, List3)</c>.</p>
        <p><em>Examples:</em></p>
        <pre>
> <input>lists:zipwith3(fun(X, Y, Z) -> X+Y+Z end, [1,2,3], [4,5,6], [7,8,9]).</input>
[12,15,18]
> <input>lists:zipwith3(fun(X, Y, Z) -> [X,Y,Z] end, [a,b,c], [x,y,z], [1,2,3]).</input>
[[a,x,1],[b,y,2],[c,z,3]]</pre>
      </desc>
    </func>
  </funcs>
</erlref>

