theorem Th4:
  divs(y,x,X,Inv) is surreal-membered
proof
  let o;
  assume o in divs(y,x,X,Inv);
  then ex xL be object st
    xL in X & xL <>0_No & o = (1_No +'(xL +' -' x) *' y) *' (Inv.xL) by Def2;
  hence thesis;
end;
