(** A library of positive multivariate polynomials *)
(** * Representation of polynomials:  - [pow] is the type of a variable identifier and its exponent.  - [mon] is the type of a monomial, consisting of a coefficient and a list of variable identifiers with their exponents.  - [pol] is the type of a polynomial, consisting of the number of variables and a list of monomials. *)
Definition pow : Type := (nat*nat)%type.
Definition mon : Type := (nat * list pow)%type.
Definition pol : Type := (nat * list mon)%type.
(** * Evaluation of polynomials  - [peval p [v0, v1,...]] is the evaluation of the polynomial [p] with variables [0], [1]... respectively instantiated with the values [v0], [v1]... If a variable is not assigned a value, then it is assumed to be [0]. Values that do not correspond to a variable are ignored. *)
Definition peval_pow (xn:pow)(l:list nat) : nat :=
power (nth (fst xn) l 0) (snd xn).
Definition peval_mon (m:mon)(l:list nat) : nat :=
(fst m) * multl (map (fun x => peval_pow x l) (snd m)).
Definition peval (p:pol)(l:list nat) :=
plusl (map (fun m => peval_mon m l) (snd p)).

Lemma peval_nth i pl p l :
peval (nth i pl p) l =
nth i (map (fun p => peval p l) pl) (peval p l).