Problem 3 of Homework 2 seems to have been harder than expected (there was a slight inaccuracy in its formulation, which may have somewhat contributed to this difficulty). Here is a reasonably detailed solution to this problem. It looks longer than it actually is, since I spend some time trying to explain where the requirements we use come from.
(There is a small typo on page 5, line 6. Instead of `sup’ it should be `union.’)
A kind of library 