Set Implicit Arguments.
Unset Strict Implicit.
Unset Printing Implicit Defensive.
Open Scope nat_scope.
Definition sum_of_two_square :=
[qualify a x |
[exists a : 'I_x.+1, exists b : 'I_x.+1, x == a ^ 2 + b ^ 2]].
Fact sum2s0 : 0 \is a sum_of_two_square.

Lemma modn_prod I r (P : pred I) F d :
\prod_(i <- r | P i) (F i %% d) = \prod_(i <- r | P i) F i %[mod d].