⊦ str ' ∈ dnode(md, sdt) ↔ str ∈ sdt
str ' ∈ tree ⊦ ∃ md, sdt. tree = dnode(md, sdt) ∧ str ∈ sdt
Used simplifier rules: one-ind: ⊦ str ' ∈ dnode(md, sdt) ↔ str ∈ sdt;