fact: Definition combine_mx (a b c d : R) (m : nat) (k : 'I_m) := let k' := lift 0 k in let d := \row_j (a *+ (j == 0) + d *+ (j == k') + ((j != 0) && (j != k'))%:R) in diag_mx d + c *: delta_mx k' 0 + b *: delta_mx 0 k'.
imports: HB(structures) Coq(ssreflect ssrfun ssrbool Arith.Wf_nat) mathcomp(eqtype ssrnat div seq path) mathcomp(ssralg fintype perm tuple choice generic_quotient) mathcomp(matrix bigop zmodp mxalgebra poly) stronglydiscrete