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'
  H0 : #|a :: r| = #|a0 :: r'|
  H : (l ++ [a]) ++ r = (l' ++ [a0]) ++ r'
  ============================
   (#|r| = #|r'| → #|r| = #|r'|)


DEBUG:Going to execute:
do_atom