(** **** Exercise: 3 stars, recommended (beq_natP_practice)  *)
(** Use [beq_natP] as above to prove the following: *)
Fixpoint count n l :=
match l with
| [] => 0
| m :: l' => (if beq_nat n m then 1 else 0) + count n l'
end.

Theorem beq_natP_practice : forall n l,
count n l = 0 -> ~(In n l).