Goal:
  
  A : Type
  a : A
  l1 : list A
  x : A
  y : A
  IHl1 : ∀ l2 : list A, l1 ++ [x] = l2 ++ [y] → l1 = l2 ∧ x = y
  a0 : A
  l2 : list A
  H : a :: l1 ++ [x] = (a0 :: l2) ++ [y]
  H1 : a = a0
  H2 : l1 ++ [x] = l2 ++ [y]
  ============================
   (a0 :: l1 = a0 :: l2 ∧ x = y)


DEBUG:Going to execute:
subst