theorem Th3:
  (F /\/ RD).(Class(RD,a), Class(RD,b)) = Class(RD, F.(a,b))
proof
A1: b in EqClass(RD,b) by EQREL_1:20;
  a in EqClass(RD,a) by EQREL_1:20;
  hence thesis by A1,Def4;
end;
