Proof
fun x y H
=> eq_sym
(f_equal {-} H
|| a = - y @a by <- op_cancel_neg x).

(** Proves that the left inverse of x + y is -y + -x. *)
Theorem op_neg_distrib_inv_l
:  forall x y : E, op_is_inv_l (x + y) (- y + - x).