fact: Definition prod_types := [eqType of {dffun forall i : 'I_n, if i \in I then types i else unit_finType}].
imports: mathcomp(all_ssreflect ssralg ssrnum matrix) mathcomp(Rstruct) Reals ssrR Reals_ext logb ssr_ext ssralg_ext bigop_ext fdist proba