Goal:
  
  A : Type
  size : A → nat
  a : A
  l : list A
  l' : list A
  IHl : list_size (l ++ l') = list_size l + list_size l'
  ============================
   (S (size a + list_size (l ++ l')) =
    S (size a + list_size l + list_size l'))


DEBUG:Going to execute:
rewrite IHl