reserve X,x,y,z for set;
reserve n,m,k,k9,d9 for Nat;
reserve d for non zero Nat;
reserve i,i0,i1 for Element of Seg d;

theorem Th3:
  for x,y being object holds {x,y} is trivial iff x = y
proof
  let x,y be object;
  hereby
A1: x in {x,y} by TARSKI:def 2;
    y in {x,y} by TARSKI:def 2;
    hence {x,y} is trivial implies x = y by A1;
  end;
  {x,x} = {x} by ENUMSET1:29;
  hence thesis;
end;
