⊦ fids = fids0 ↔ (∀ fid. fid ∈ fids ↔ fid ∈ fids0)
⊦ fd ∈ ofh, ∀ fid. fid ∈ ofh[fd, fh].fids ↔ fid ∈ ofh.fids ++ fh.fid
Used terms: ofh.fids ++ fh.fid, ofh[fd, fh].fids