Goal:
  
  A : Type
  l : list A
  l' : list A
  a : A
  r : list A
  a0 : A
  r' : list A
  IHr : ∀ l l' r' : list A,
          l ++ r = l' ++ r' → #|r| = #|r'| → l = l' ∧ r = r'
  H : l ++ a :: r = l' ++ a0 :: r'
  H0 : #|a :: r| = #|a0 :: r'|
  ============================
   (l = l' ∧ a :: r = a0 :: r')


DEBUG:Going to execute:
discriminate