How to prove that another definition of permutation is the same as the Default Permutation Library for COQ

Question

I need to proove that a secondary definition of permutation is equivalent to the default definition of permutation in Coq:

Down bellow is the default Permutation definition in Coq

Inductive Permutation : list A -> list A -> Prop :=
| perm_nil: Permutation [] []
| perm_skip x l l' : Permutation l l' -> Permutation (x::l) (x::l')
| perm_swap x y l : Permutation (y::x::l) (x::y::l)
| perm_trans l l' l'' :
    Permutation l l' -> Permutation l' l'' -> Permutation l l''.

I need to prove that the above mentioned definition is equivalent to the following definition:

Definition perm l l' := forall x, occurences_number x l = occurences_number x l'.

Which as you have noticed uses the definition occurences_number down bellow:

Fixpoint occurences_number x l := match l with
                       | nil => 0
                       | h::tl => if (x =? h) then S(occurences_number x tl) else occurences_number x tl
                       end.

What I need to prove indeed is the following:

Lemma permutation_to_perm: forall l l', Permutation l l' -> perm l l'.

Down bellow is my incomplete proof

Proof.
 induction l.
 - admit.
 - intros l' Hequiv.
   generalize dependent a.
   generalize dependent l.
   case l'.
   + Admitted.