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
  {x} /\ {x,y} = {x}
proof
  x in {x,y} by TARSKI:def 2;
  hence thesis by Lm1,XBOOLE_1:28;
end;
