ex 4.37 - done
ex 4.37 - list monad vs. amb
4.3.2 - examples of nondeterministic programs
4.3.2 - logic puzzles - list monad as amb
ex 4.38 - done
ex 4.39 - explanation
ex 4.39 - the order of constraint matters
ex 4.39 - code explained
ex 4.39 - done