theorem Th21:
  for a being Element of PFuncs (V, C) st a is finite for c being
  set st c in Atom(V, C).a holds c = a
proof
  let a be Element of PFuncs (V, C);
  assume a is finite;
  then Atom(V, C).a = { a } by Lm7;
  hence thesis by TARSKI:def 1;
end;
