⊦ a ∈ s1 \ s2 ↔ a ∈ s1 ∧ ¬ a ∈ s2
a0 = a ∨ a0 ∈ s0 ∧ ¬ a0 ∈ x0.toSet ⊦ a ∈ x0, (a0 = a ∨ a0 ∈ s0) ∧ ¬ a0 ∈ x0
Used terms: x0.toSet, s0, a0