Used simplifier rules: sub-add-02: ⊦ s ++ a ⊆ s0 → s ⊆ s0;%sub-add-03: ⊦ s ++ a ⊆ s0 ↔ s ⊆ s0 ∧ a ∈ s0;%rax: ⊦ a ∈ s ++ b ↔ ¬ ¬(a = b ∨ a ∈ s);