reserve a, I for set,
  S for non empty non void ManySortedSign;

theorem Th4:
  for I being non empty set holds Class(nabla I) = {I}
proof
  let I be non empty set;
  consider a being object such that
A1: a in I by XBOOLE_0:def 1;
  thus Class(nabla I) c= {I} by Th3;
  let q be object;
  assume q in {I};
  then
A2: q = I by TARSKI:def 1;
  Class(nabla I,a) = I by A1,EQREL_1:26;
  hence thesis by A2,A1,EQREL_1:def 3;
end;
