| ⊦ str = str0, (str1 ∈ sdt[str0, tree] -- str ↔ str1 ∈ (sdt -- str)[str0, tree]) ∧ (sdt[str0, tree] -- str)[str1] = (sdt -- str)[str0, tree][str1] |
⊦ str1 ∈ sdt[str0, tree] -- str ↔ str1 ∈ (sdt -- str)[str0, tree], str = str0
⊦ (sdt[str0, tree] -- str)[str1] = (sdt -- str)[str0, tree][str1], str = str0