reserve m,n for Nat;
reserve i,j for Integer;
reserve S for non empty addMagma;
reserve r,r1,r2,s,s1,s2,t,t1,t2 for Element of S;
reserve G for addGroup-like non empty addMagma;
reserve e,h for Element of G;
reserve G for addGroup;
reserve f,g,h for Element of G;
reserve u for UnOp of G;

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;
