fact: Fixpoint augment s z x := if s is ((z', x') as d) :: s then if x == x' then (z + z', x) :: s else d :: augment s z x else [:: (z, x)].
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype bigop order generic_quotient) mathcomp(ssralg ssrnum ssrint)