Goal:
  
  X : Type
  Y : Type
  R : nat → X → Y → Type
  n : nat
  x : X
  l0 : list X
  a0 : Y
  ys : list Y
  ys' : list Y
  r0 : R n x a0
  a : All2i R (S n) l0 (ys ++ ys')
  ============================
   (let H := CoreTactics.block in
    (∀ (n : nat) (l : list X) (ys' : list Y),
       All2i R n l (ys ++ ys')
       → ∑ xs xs' : list X,
           l = xs ++ xs' × All2i R n xs ys × All2i R (n + #|xs|) xs' ys')
    → let H0 := CoreTactics.block in
      ∑ xs xs' : list X,
        x :: l0 = xs ++ xs'
        × All2i R n xs (a0 :: ys) × All2i R (n + #|xs|) xs' ys')


DEBUG:Going to execute:
change T; CoreTactics.intros_until_block
Level 3: T
 has value 
((∀ (n : nat) (l : list X) (ys' : list Y),
    All2i R n l (ys ++ ys')
    → ∑ xs xs' : list X,
        l = xs ++ xs' × All2i R n xs ys × All2i R (n + #|xs|) xs' ys')
 → let H := CoreTactics.block in
   ∑ xs xs' : list X,
     x :: l0 = xs ++ xs'
     × All2i R n xs (a0 :: ys) × All2i R (n + #|xs|) xs' ys')