theorem Th19:
  for u being Element of SubstitutionSet (V, C) holds u = FinJoin(
  u, Atom (V, C))
proof
  let u be Element of SubstitutionSet (V, C);
  thus u = FinUnion(u, singleton PFuncs (V, C)) by SETWISEO:58
    .= FinJoin(u, Atom(V, C)) by Th18;
end;
