where <math> is the prefix length.
spanl,
applied to a predicate
p and a sequence
xs, returns
a
These based on the point where the predicate fails:
- This ys means that the predicate was true for all
items, and ys is the entire original sequence.
- That zs means that the predicate failed on the
first item, and zs is the entire original sequence.
- These ys zs gives ys (the prefix of
elements that satisfy the predicae) and zs (the remainder of
the sequence)