a ∈ s0, a1 ∈ s0 ⊦ a0 ∈ s \ s0 ↔ a0 ∈ s \ (s0 -- a), a ∈ s
Used simplifier rules: ⊦ ¬ a ∈ ∅; (s)%ne-01: ⊦ a ∈ s → s ≠ ∅; (forward) from specification set-basic% ⊦ s1 = s2 ↔ (∀ a. a ∈ s1 ↔ a ∈ s2); (s)