reserve u,v,x,x1,x2,y,y1,y2,z,p,a for object,
        A,B,X,X1,X2,X3,X4,Y,Y1,Y2,Z,N,M for set;

theorem Th4:
  {x} = {y1,y2} implies x = y1
proof
  assume { x } = { y1,y2 };
  then y1 in { x } by TARSKI:def 2;
  hence thesis by TARSKI:def 1;
end;
