theorem     :: Tarski:4 b
  X = { x1,x2 } implies for x holds x = x1 or x = x2 implies x in X
proof
  assume
A1: X = { x1,x2 };
  let x;
  assume
A2: x = x1 or x = x2;
  let i;
  assume i in I;
  then
A3: X.i = {x1.i,x2.i} by A1,Def2;
  per cases by A2;
  suppose x = x1;
    hence thesis by A3,TARSKI:def 2;
  end;
  suppose x = x2;
    hence thesis by A3,TARSKI:def 2;
  end;
end;
