(* pi- parts *)
(* Testing for membership in set of prime factors. *)
Coercion nat_pred_of_nat (p : nat) : nat_pred := pred1 p.
Section NatPreds.
Variables (n : nat) (pi : nat_pred).
Definition negn : nat_pred := [predC pi].
Definition pnat : pred nat := fun m => (m > 0) && all (mem pi) (primes m).
Definition partn := \prod_(0 <= p < n.+1 | p \in pi) p ^ logn p n.
End NatPreds.
Notation "pi ^'" := (negn pi) (at level 2, format "pi ^'") : nat_scope.
Notation "pi .-nat" := (pnat pi) (at level 2, format "pi .-nat") : nat_scope.
Notation "n `_ pi" := (partn n pi) : nat_scope.
Section PnatTheory.
Implicit Types (n p : nat) (pi rho : nat_pred).

Lemma filter_pi_of n m : n < m -> filter \pi(n) (index_iota 0 m) = primes n.