⊦ (∅ -- str)[str0] = (∅)[str0], str0 ∈ ∅ -- str
Used simplifier rules: ⊦ str ∈ sdt -- str0 ↔ str ≠ str0 ∧ str ∈ sdt; (ws)% ⊦ ¬ str ∈ ∅; (s)