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
  H : n <= m
  z0 : Z
  H0 : m < a /\ z0 = a \/ a <= m /\ z0 = m
  ============================
   (n <= z0)


DEBUG:Going to execute:
xlia zchecker