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,y} c= {z} implies {x,y} = {z}
proof
  assume {x,y} c= {z};
  then x=z & y=z by Th20;
  hence thesis by ENUMSET1:29;
end;
