fact: Fixpoint merge s1 := if s1 is x1 :: s1' then let fix merge_s1 s2 := if s2 is x2 :: s2' then if leT x1 x2 then x1 :: merge s1' s2 else x2 :: merge_s1 s2' else s1 in merge_s1 else id.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq)