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-This problem is taken from _The Craft of Functional Programming_ by Simon Thompson, Addison-Wesley 1999 <http://www.cs.kent.ac.uk/people/staff/sjt/>:
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-> 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
-> an as-yet-unvisited element we have to find a "new" number to match
-> it with.
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-Here is [a hint](/hints/assignment_10_hint).
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