Goal:
  
  A : Type
  a : A
  l : list A
  l' : list A
  IHl : ∀ l' : list A, rev (l ++ l') = rev l' ++ rev l
  ============================
   ((rev l' ++ rev l) ++ [a] = (rev l' ++ rev l) ++ [a])


DEBUG:Going to execute:
reflexivity
DEBUG LTAC Evaluated term: app_assoc
DEBUG LTAC Evaluated term: app_assoc