a ∈ st, ∀ a0. a0 ∈ st -- a ↔ a0 ∈ st ⊦
Used simplifier rules: notin-del-notin: ⊦ ¬ a ∈ st → st -- a = st;%Del-notin: ⊦ ¬ a ∈ st -- a;%Del-at-strong: a ≠ b ⊦ (st -- b)[a] = st[a];