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


DEBUG:Going to execute:
discriminate