O(n).
mapMonotonic f s == map f s, but works only when f is strictly increasing.
Semi-formally, we have:
and [x < y ==> f x < f y | x <- ls, y <- ls]
==> mapMonotonic f s == map f s
where ls = toList sWarning: This function should be used only if f is monotonically
strictly increasing. This precondition is not checked. Use map if the
precondition may not hold.