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

theorem
  born (x+y) c= born x (+) born y
proof
  x+y in Day (born x (+) born y) by Lm5;
  hence thesis by SURREAL0:def 18;
end;
