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

theorem Th59:
  for X,Y1,Y2 be set st Y1 c= Y2 holds comp(X,x,y,Y1) c= comp(X,x,y,Y2)
proof
  let X,Y1,Y2 be set such that A1: Y1 c= Y2;
  let o;
  assume o in comp(X,x,y,Y1);
  then ex x1,y1 be Surreal st
  o = (x1*y) +' (x*y1) +' -' (x1*y1) & x1 in X & y1 in Y1 by Def14;
  hence thesis by A1,Def14;
end;
