Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  l : list A
  a : B
  r1 : list B
  r2 : list B
  h1' : list A
  h1 : list B
  ============================
   (All2 R h1' h1
    → {| pr1 := l; pr2 := a :: r1 ++ r2 |} = {| pr1 := h1'; pr2 := h1 |}
      → let H := CoreTactics.block in
        (∀ (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 H0 := CoreTactics.block in
          ∑ l1 l2 : list A, l = l1 ++ l2 × All2 R l1 (a :: r1) × All2 R l2 r2)


DEBUG:Going to execute:
intros p