Goal:
  
  A : Type
  H : ReflectEq A
  a1 : A
  l1 : list A
  a2 : A
  l2 : list A
  Heq : (a1 == a2) = false
  ============================
   (∀ l3 l4 : list A,
      {| pr1 := a1 :: l1; pr2 := a2 :: l2 |} = {| pr1 := l3; pr2 := l4 |}
      → ([], a1 :: l1, a2 :: l2) = split_prefix l3 l4
        → let H := CoreTactics.block in
          let
          '(prefix, l1', l2') := split_prefix l3 l4 in
           l3 = prefix ++ l1' ∧ l4 = prefix ++ l2')


DEBUG:Going to execute:
intro H