Goal:
  
  n : Z
  a : Z
  l : list Z
  IHl : forall m : Z, n <= m \/ Exists (Z.le n) l -> n <= fold_left Z.max l m
  m : Z
  H0 : n <= a
  ============================
   (n <= Z.max m a)


DEBUG:Going to execute:
zify_op