-- > plus n x = execState (sequence $ replicate n tick) x
--
-- An example from /The Craft of Functional Programming/, Simon
--- Thompson (<http://www.cs.ukc.ac.uk/people/staff/sjt/>),
--- Addison-wesley 1999: \"Given an arbitrary tree, transform it to a
+-- Thompson (<http://www.cs.kent.ac.uk/people/staff/sjt/>),
+-- Addison-Wesley 1999: \"Given an arbitrary tree, transform it to a
-- tree of integers in which the original elements are replaced by
-- natural numbers, starting from 0. The same element has to be
-- replaced by the same number at every occurrence, and when we meet