(** * Lists of natural numbers. *)

Definition natlist := list nat.(** Multiply all elements of a natlist *)

Fixpoint product (qlist : natlist) : nat :=
  match qlist with
  | Nil => 1
  | Cons m l => m * product l
  end.(** Drop the first occurance of q from qlist *)

Fixpoint drop (q : nat) (qlist : natlist) {struct qlist} : natlist :=
  match qlist with
  | Nil => Nil nat
  | Cons q' l => if beq_nat q q' then l else Cons nat q' (drop q l)
  end.(** Multiply all elements of qlist, except for (first occurance of) q *)

Definition multDrop (q : nat) (l : natlist) := product (drop q l).

Lemma multdrop_cons_neq : forall (p q : nat) (l : natlist), p <> q -> multDrop p (Cons nat q l) = q * multDrop p l.