fact: Fixpoint schange_index_aux l i y := if l is x::l' then if (((y==0) && (x != 0)) || (x*y < 0)) then i :: schange_index_aux l' i.+1 x else schange_index_aux l' i.+1 y else [::].
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice fintype order) mathcomp(binomial bigop ssralg poly ssrnum ssrint rat) mathcomp(polyrcf) pol