a ∈ s ⊦ s = ∅, st[a, d][b] = st[b], # s = 0, b ∈ s
Used simplifier rules: size-zero-01: ⊦ s ≠ ∅ → # s ≠ 0; (forward) from specification set-basic%ne-01: ⊦ a ∈ s → s ≠ ∅; (forward) from specification set-basic