reserve A,B,C for Ordinal,
        o for object,
        x,y,z,t,r,l for Surreal,
        X,Y for set;

theorem
  for x1,x2,y1,y2 be Surreal st x1 < x2 & y1 < y2
    holds x1*y2+x2*y1 < x1*y1+x2*y2 by Th51;
