Goal:
  
  G : t
  s0 : VSet.t
  x : V.t
  e : x = x
  XX : forall (l : list Nbar.t) (acc : Nbar.t),
       (Some 0 <= acc)%nbar -> (Some 0 <= fold_left max l acc)%nbar
  ============================
   (Some 0 <= (if is_left (left e) then Some 0 else None))%nbar


DEBUG:Going to execute:
cbn
DEBUG LTAC Evaluated term: XX