theorem
  for f9,g9 being Function st dom f9 misses dom g \ dom g9 &
  f9 in sproduct f & g9 in sproduct g holds f9+*g9 in sproduct(f+*g)
proof
  let f9,g9 be Function;
  assume dom f9 misses dom g \ dom g9;
  then dom g misses dom f9 \ dom g9 by XBOOLE_1:81;
  hence thesis by Th68;
end;
