:: Ord a => [a] -> [a] package:lens is:exact
returns the elements of xs
reverse order. xs
must be finite.
ties a finite list into a circular one, or equivalently,
the infinite repetition of the original list. It is the identity on
Return all the elements of a list except the last one.
The list must be non-empty.
Extract the elements after the head of a list, which
must be non-empty.