a ∈ s2 ⊦ a0 ∈ s1 \ s2 ↔ a0 ∈ s1 \ (s2 -- a), a ∈ s1 \ s2, a ∈ s1
Used simplifier rules: del-in: ⊦ ¬ a ∈ s → s -- a = s; Used formulas: a ∈ s2