
theorem Th1:
for x being object, X being set holds not [x,X] in X
proof
  let x be object, X be set;
  A1: X in {x, X} by TARSKI:def 2;
  {x, X} in {{x, X}, {x}} by TARSKI:def 2;
  hence thesis by A1,XREGULAR:7;
end;
