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