fact: Definition add_RV (X Y : {RV P -> R}) : {RV P -> R} := fun x => X x + Y x.
imports: mathcomp(all_ssreflect ssralg ssrnum matrix) mathcomp(Rstruct reals) Reals Lra ssrR Reals_ext realType_ext logb ssr_ext ssralg_ext bigop_ext fdist