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 Th6:
  { x1,x2 } = { y1,y2 } implies x1 = y1 or x1 = y2
proof
  x1 in { x1,x2 } by TARSKI:def 2;
  hence thesis by TARSKI:def 2;
end;
