(* (c) Copyright 2006-2016 Microsoft Corporation and Inria.                  *)
(* Distributed under the terms of CeCILL-B.                                  *)
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(* This files contains the definition of:                                     *)
(*   'C(n, m) == the binomial coefficient n choose m.                         *)
(*     n ^_ m == the falling (or lower) factorial of n with m terms, i.e.,    *)
(*               the product n * (n - 1) * ... * (n - m + 1).                 *)
(*               Note that n ^_ m = 0 if m > n, and 'C(n, m) = n ^_ m %/ m`!. *)
(*                                                                            *)
(* In additions to the properties of these functions, we prove a few seminal  *)
(* results such as triangular_sum, Wilson and Pascal; their proofs are good   *)
(* examples of how to manipulate expressions with bigops.                     *)
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Set Implicit Arguments.
Unset Strict Implicit.
Unset Printing Implicit Defensive.
(** More properties of the factorial **)
(** The falling factorial *)
Fixpoint ffact_rec n m := if m is m'.+1 then n * ffact_rec n.-1 m' else 1.
Definition falling_factorial := nosimpl ffact_rec.
Notation "n ^_ m" := (falling_factorial n m)
(at level 30, right associativity) : nat_scope.

Lemma ffact_factd n m : m <= n -> n ^_ m = n`! %/ (n - m)`!.