theorem Th21:
  for G being add-unital non empty addMagma holds the_unity_wrt the
  addF of G = 0_G
proof
  let G be add-unital non empty addMagma;
  0_G is_a_unity_wrt the addF of G by Th20;
  hence thesis by BINOP_1:def 8;
end;
