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
  {x1,x2} c= {y1,y2} implies x1 = y1 or x1 = y2
proof
  assume {x1,x2} c= {y1,y2};
  then {x1,x2}={} or {x1,x2}={y1} or {x1,x2}={y2} or {x1,x2}={y1,y2} by Lm13;
  hence thesis by Th4,Th6;
end;
