fact: Fixpoint lt s1 s2 := if s2 isn't x2 :: s2' then false else if s1 isn't x1 :: s1' then true else (x1 <= x2) && ((x1 >= x2) ==> lt s1' s2').
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(path fintype tuple bigop finset div prime finfun) mathcomp(finset)