Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  a : A
  l1 : list A
  IHl1 : ∀ (l2 : list A) (r : list B),
           All2 R (l1 ++ l2) r
           → ∑ r1 r2 : list B, r = r1 ++ r2 × All2 R l1 r1 × All2 R l2 r2
  l2 : list A
  y : B
  l' : list B
  r0 : R a y
  X : All2 R (l1 ++ l2) l'
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    ∑ r1 r2 : list B, y :: l' = r1 ++ r2 × All2 R (a :: l1) r1 × All2 R l2 r2)


DEBUG:Going to execute:
change T; CoreTactics.intros_until_block
Level 3: T
 has value 
(let H := CoreTactics.block in
 ∑ r1 r2 : list B, y :: l' = r1 ++ r2 × All2 R (a :: l1) r1 × All2 R l2 r2)