Support linear sequence search patterns

e.g.
```python
[1, 2] + xs + [5, 6] + ys + [9, 10] = range(1, 11)
"a:" + a + ",b:" + b = "a:1,b:1"

def first_twin(_ + [p, (.-2) -> p] + _) = (p, p+2)
def first_twin_((*_, p, (.-2) -> p, *_)) = (p, p+2)

def has_abc(s):
    match _ + "abc" + _ in s:
        return True
    else:
        return False
```