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

theorem Th43:
  x <= y & z <= t implies x+z <= y+t
proof
  assume x <= y & z <= t;
  then x+z <= y+z & y+z <= y+t by Th32;
  hence thesis by SURREALO:4;
end;
