Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  l : list A
  a : B
  r1 : list B
  r2 : list B
  h : All2 R l ((a :: r1) ++ r2)
  IHr1 : ∀ (l : list A) (r2 : list B),
           All2 R l (r1 ++ r2)
           → ∑ l1 l2 : list A, l = l1 ++ l2 × All2 R l1 r1 × All2 R l2 r2
  ============================
   (let H := CoreTactics.block in
    ∑ l1 l2 : list A, l = l1 ++ l2 × All2 R l1 (a :: r1) × All2 R l2 r2)


DEBUG:Going to execute:
fun tac => lazymatch goal with
                      | H:_ |- _ => tac H
                      end