Goal:
  
  X : Type
  Y : Type
  R : nat → X → Y → Type
  n : nat
  a0 : X
  xs : list X
  xs' : list X
  y : Y
  r : list Y
  r0 : R n a0 y
  a : All2i R (S n) (xs ++ xs') r
  ============================
   (let H := CoreTactics.block in
    (∀ (n : nat) (xs' : list X) (l : list Y),
       All2i R n (xs ++ xs') l
       → ∑ ys ys' : list Y,
           l = ys ++ ys' × All2i R n xs ys × All2i R (n + #|xs|) xs' ys')
    → let H0 := CoreTactics.block in
      ∑ ys ys' : list Y,
        y :: r = ys ++ ys'
        × All2i R n (a0 :: xs) ys × All2i R (n + #|a0 :: xs|) xs' ys')


DEBUG:Going to execute:
change T; CoreTactics.intros_until_block
Level 3: T
 has value 
((∀ (n : nat) (xs' : list X) (l : list Y),
    All2i R n (xs ++ xs') l
    → ∑ ys ys' : list Y,
        l = ys ++ ys' × All2i R n xs ys × All2i R (n + #|xs|) xs' ys')
 → let H := CoreTactics.block in
   ∑ ys ys' : list Y,
     y :: r = ys ++ ys'
     × All2i R n (a0 :: xs) ys × All2i R (n + #|a0 :: xs|) xs' ys')