fact: Definition GRS_mod r : {poly F} := \sum_(i in supp y) y ``_ i *: (\prod_(i0 in E | i0 != i) ((1 - a ``_ i0 *: 'X))) * (a ``_ i ^+ r)%:P * - (b ``_ i)%:P.
imports: mathcomp(all_ssreflect ssralg finalg poly polydiv cyclic) mathcomp(perm matrix mxpoly vector mxalgebra zmodp) ssr_ext ssralg_ext vandermonde linearcode dft poly_decoding