a0 ∈ st ⊦ st[a0] = d1, a0 = a, ∃ a1. a1 ∈ st ∧ st[a1] = st[a0]
Used simplifier rules: ⊦ a ≠ b → (a ∈ st[b, d] ↔ a ∈ st); (s)% ⊦ a ≠ b → st[b, d][a] = st[a]; (s)