fact: Definition find_pivot m n (A : 'M[F]_(m,n.+1)) : option 'I_m := [pick k | A k 0 != 0].
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice fintype) mathcomp(div finfun bigop prime binomial ssralg finset fingroup finalg) mathcomp(mxalgebra perm zmodp matrix ssrint)