These predicates implement "association list" primitives. They use a
binary tree representation. Thus the time complexity for these
predicates is O(lg N), where N is the number of keys. These
predicates also illustrate the use of compare/3 (see Term Compare) for case analysis.
The goal get_assoc(Key, Assoc, Value) is true
when Key is identical to one of the keys in Assoc, and
Value unifies with the associated value.
get_assoc(Key, t(K,V,L,R), Val) :-
compare(Rel, Key, K),
get_assoc(Rel, Key, V, L, R, Val).
get_assoc(=, _, Val, _, _, Val).
get_assoc(<, Key, _, Tree, _, Val) :-
get_assoc(Key, Tree, Val).
get_assoc(>, Key, _, _, Tree, Val) :-
get_assoc(Key, Tree, Val).