reserve x,x1,x2,x3,x4,x5,x6,x7,x8,x9,x10,y for object, X,Z for set;

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;
