⊦ ps = ps0 ↔ (∀ p. p ∈ ps ↔ p ∈ ps0)
⊦ ∀ p. p ∈ links(fid, fnode(fid0)) ↔ p ∈ (fid = fid0 ⊃ {ε} ;∅)
Used terms: (fid = fid0 ⊃ {ε} ;∅), links(fid, fnode(fid0))