fact: Definition prod_of_products_canon f f':= Lf (fun w => prod_of_fgraph (P w) (Q w)) ((productb f) \times (productb f')) (productf (domain f)(fun i => (Vg f i) \times (Vg f' i))).
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)