⊦ a ∈ x.toSet ↔ a ∈ x
a0 = a ∨ a0 ∈ s0 ∧ ¬ a0 ∈ x0 ⊦ a ∈ x0, (a0 = a ∨ a0 ∈ s0) ∧ ¬ a0 ∈ x0
Used simplifier rules: toSet-ext: ⊦ a ∈ x.toSet ↔ a ∈ x;