induction l using list_ind'.
+ intros. exists f. auto.
+ intros. simpl in *. omega.
+ intros l' l'' lApp f n p Len Valid.
destruct n.
- simpl in Len. congruence.
- simpl in Len; assert (length l = (2 * n)%nat) by omega.
destruct a eqn:A; destruct b eqn:B; simpl in *.
* unfold pathToPos in Valid. simpl in Valid.
pose proof (IHl l' l'' lApp (fun p => xO (f p)) n p H Valid) as Hyp; clear IHl H.
inversion Hyp as [f' HF]; clear Hyp.
exists f'.
auto.
* unfold pathToPos in Valid. simpl in Valid.
pose proof (IHl l' l'' lApp (fun p => xI (f p)) n p H Valid) as Hyp; clear IHl H.
inversion Hyp as [f' HF]; clear Hyp.
exists f'.
auto.
* inversion Valid.
* inversion Valid.
Qed.

(* Specifications: *)
Lemma pathBeginsLeft : forall l1 l2 f x, l1 <> [] -> l2 <> [] ->
pathToPosAux (l1 ++ l2) f = Some x ->
head l1 = Some Left.