fails:Array#combination returns an enumerator when no block is provided
fails:Array#combination returns self when a block is given
fails:Array#combination yields nothing for out of bounds length and return self
fails:Array#combination yields the expected combinations
fails:Array#combination yields nothing if the argument is out of bounds
fails:Array#combination yields a copy of self if the argument is the size of the receiver
fails:Array#combination yields [] when length is 0
fails:Array#combination yields a partition consisting of only singletons
fails:Array#combination generates from a defensive copy, ignoring mutations
