theorem Th20:
  for G being add-unital non empty addMagma holds 0_G is_a_unity_wrt
  the addF of G
proof
  let G be add-unital non empty addMagma;
  set o = the addF of G;
  now
    let h be Element of G;
    thus o.(0_G,h) = 0_G + h .= h by Def4;
    thus o.(h,0_G) = h + 0_G .= h by Def4;
  end;
  hence thesis by BINOP_1:3;
end;
