theorem Th29:
  { x1,x1 } = { x1 }
proof
  now
    let x be object;
    x in { x1,x1 } iff x = x1 by TARSKI:def 2;
    hence x in { x1,x1 } iff x in { x1 } by TARSKI:def 1;
  end;
  hence thesis by TARSKI:2;
end;
