Used simplifier rules: del-addeq-05: ⊦ (s ++ a) -- a = s -- a;%del-e-01: ⊦ ∅ -- a = ∅;%ext: ⊦ s1 = s2 ↔ (∀ a. a ∈ s1 ↔ a ∈ s2);