reserve m for Cardinal,
  A,B,C for Ordinal,
  x,y,z,X,Y,Z,W for set,
  f for Function;

theorem Th3:
  W is Tarski & x in W & y in W implies [x,y] in W
proof
  assume
A1: W is Tarski;
  assume that
A2: x in W and
A3: y in W;
A4: {x} in W by A1,A2,Th2;
  {x,y} in W by A1,A2,A3,Th2;
  hence thesis by A1,A4,Th2;
end;
