fact: Definition sum_2 := X =1 fun x => X1 (x ``_ ord0) + X2 (rbehead 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