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

theorem Th3:
  Class(nabla I) c= {I}
proof
  let q be object;
  assume q in Class(nabla I);
  then consider x being object such that
A1: x in I and
A2: q = Class(nabla I,x) by EQREL_1:def 3;
  Class(nabla I,x) = I by A1,EQREL_1:26;
  hence thesis by A2,TARSKI:def 1;
end;
