fact: Definition mxdiag (B_ : forall i, 'M[V]_(p_ i)) : 'M[V]_(\sum_i p_ i) := \mxblock_(j, k) if j == k then conform_mx 0 (B_ j) else 0.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype finfun bigop finset fingroup perm order) mathcomp(div prime binomial ssralg countalg finalg zmodp)