diff --git a/fz/util.pl b/fz/util.pl
new file mode 100644
index 0000000..4fcbeda
--- /dev/null
+++ b/fz/util.pl
@@ -0,0 +1,12 @@
+% == No, Emacs this is -*-Prolog-*- code, not what you thought... =============
+
+% -- utility predicates -------------------------------------------------------
+
+% -- subset (SUB, SET, REST) --------------------------------------------------
+    
+subset(L, L, []).
+subset(S, [H|T], R) :- subset_aux(T, H, S, R).
+
+   subset_aux(S, R, S, [R]).
+   subset_aux([H|T], R1, S, [R1|R]) :- subset_aux(T, H, S, R).
+   subset_aux([H|T], X, [X|S], R) :- subset_aux(T, H, S, R).
--
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