Used simplifier rules: del-feq: ⊦ s -- a = s0 ++ b → a ≠ b ∧ b ∈ s ∧ ¬ a ∈ s0;%in-03: ⊦ ¬ a ∈ s → (a ∈ s ++ b ↔ a = b); from specification set-basic