theorem
  vf (x-term) = {x}
  proof
A1: rng (x-term) = {[x,s]} by FUNCOP_1:8;
    rng (x-term) c= [:Union X, the carrier of S:] by A1,ZFMISC_1:31,87;
    then rng (x-term) /\ [:Union X, the carrier of S:] = rng (x-term)
    by XBOOLE_1:28;
    hence thesis by A1,RELAT_1:9;
  end;
