src/Pure/library.ML

 infix subset;
 fun [] subset ys = true
   | (x :: xs) subset ys = x mem ys andalso xs subset ys;
 
 fun gen_subset eq (xs, ys) = forall (fn x => gen_mem eq (x, ys)) xs;
+
+
+(*eq_set*)
+
+fun eq_set (xs, ys) =
+  xs = ys orelse (xs subset ys andalso ys subset xs);
 
 
 (*removing an element from a list WITHOUT duplicates*)
 infix \;
 fun (y :: ys) \ x = if x = y then ys else y :: (ys \ x)