:: [Int] -> Int -package:protolude package:LambdaHack
A version specialized to lists to avoid errors such as taking length
of Maybe [a] instead of [a]. Such errors are hard to
detect, because the type of elements of the list is not constrained.
Extract the last element of a list, which must be finite
and non-empty.
Extract the first element of a list, which must be
non-empty.