Goal:
  
  A : Type
  a : A
  x : A
  l : list A
  IHl : l ≠ [] → l = remove_last l ++ [last l a]
  H : l ++ [x] ≠ []
  ============================
   (l ++ [x] = l ++ [x])


DEBUG:Going to execute:
trivial; hnf; intros