test(tests/lean): add example showing how to 'sort' argumentes of AC operators using the simplifier

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

tests/lean/simp7.lean

@@ -0,0 +1,10 @@
+variables a b c d e f : Bool
+rewrite_set simple
+add_rewrite and_assoc and_comm and_left_comm : simple
+(*
+local t  = parse_lean("(f ∧ e) ∧ (d ∧ c) ∧ (b ∧ a)")
+local t2 = simplify(t, "simple")
+print(t)
+print("====>")
+print(t2)
+*)

tests/lean/simp7.lean.expected.out

@@ -0,0 +1,11 @@
+  Set: pp::colors
+  Set: pp::unicode
+  Assumed: a
+  Assumed: b
+  Assumed: c
+  Assumed: d
+  Assumed: e
+  Assumed: f
+(f ∧ e) ∧ (d ∧ c) ∧ b ∧ a
+====>
+a ∧ b ∧ c ∧ d ∧ e ∧ f