
theorem Th3:
for x, X being object holds [x,X] <> x
proof
 let x, X be object;
 assume
A1: [x,X] = x;
  reconsider x as set by TARSKI:1;
  {x,X} in x by TARSKI:def 2,A1;
 hence contradiction by TARSKI:def 2;
end;
