intros...
assert (a + - a = a + 0)...
assert (a + - a = 0)...
rewrite H1 in H0...
assert (a + 0 = a)...
Qed.

(* Specifications: *)
Lemma field_prop_5 : forall a b : F, - a = - b -> a = b.
Proof with try solve [ ring | congruence ].