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