Goal:
  
  X : Type
  Y : Type
  R : nat → X → Y → Type
  n : nat
  l : list X
  a0 : Y
  ys : list Y
  ys' : list Y
  a2' : nat
  a2'0 : list X
  a2 : list Y
  ============================
   (All2i R a2' a2'0 a2
    → {| pr1 := n; pr2 := {| pr1 := l; pr2 := a0 :: ys ++ ys' |} |} =
      {| pr1 := a2'; pr2 := {| pr1 := a2'0; pr2 := a2 |} |}
      → 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,
            l = xs ++ xs'
            × All2i R n xs (a0 :: ys) × All2i R (n + #|xs|) xs' ys')


DEBUG:Going to execute:
intros p