Proof using.
induction l; intros; simpl in *; repeat break_match; subst; simpl in *;
intuition (eauto; congruence).
Qed.
Definition get_op_input_keys (l : list op) : list K :=
filterMap (fun x => match x with
| I k => Some k
| _ => None
end) l.

(* Specifications: *)
Lemma get_op_input_keys_defn :
forall x l,
get_op_input_keys (x :: l) = match x with
| I k => k :: get_op_input_keys l
| _ => get_op_input_keys l
end.