remove vestigial prolog/ directory (#444)

src/lib/pairs.pl

@@ -0,0 +1,33 @@
+:- module(pairs, [pairs_keys_values/3,
+		  pairs_keys/2,
+		  pairs_values/2,
+		  group_pairs_by_key/2,
+		  map_list_to_pairs/3]).
+
+
+pairs_keys_values([], [], []).
+pairs_keys_values([A-B|ABs], [A|As], [B|Bs]) :-
+        pairs_keys_values(ABs, As, Bs).
+
+pairs_keys(Ps, Ks) :- pairs_keys_values(Ps, Ks, _).
+
+pairs_values(Ps, Vs) :- pairs_keys_values(Ps, _, Vs).
+
+map_list_to_pairs(Pred, Ls, Ps) :-
+        map_list_to_pairs2(Ls, Pred, Ps).
+
+map_list_to_pairs2([], _, []).
+map_list_to_pairs2([H|T0], Pred, [K-H|T]) :-
+        call(Pred, H, K),
+        map_list_to_pairs2(T0, Pred, T).
+
+
+group_pairs_by_key([], []).
+group_pairs_by_key([K-V|KVs0], [K-[V|Vs]|KVs]) :-
+        same_key(K, KVs0, Vs, KVs1),
+        group_pairs_by_key(KVs1, KVs).
+
+same_key(K0, [K1-V|KVs0], [V|Vs], KVs) :-
+        K0 == K1, !,
+        same_key(K0, KVs0, Vs, KVs).
+same_key(_, KVs, [], KVs).