⊦ fid ∈ tree.fids ↔ (∃ p. p ∈ tree ∧ tree[p] = fnode(fid))
str ∈ sdt, ∃ p. p ∈ sdt[str] ∧ sdt[str][p] = fnode(fid) ⊦ fid ∈ sdt.fids
Used terms: sdt[str], fid