Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  x : A
  l0 : list A
  a : B
  r1 : list B
  r2 : list B
  r : R x a
  h : All2 R l0 (r1 ++ r2)
  ============================
   (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,
        x :: l0 = l1 ++ l2 × All2 R l1 (a :: r1) × All2 R l2 r2)


DEBUG:Going to execute:
change T; CoreTactics.intros_until_block
Level 3: T
 has value 
((∀ (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, x :: l0 = l1 ++ l2 × All2 R l1 (a :: r1) × All2 R l2 r2)