Goal:
  
  X : Type
  Y : Type
  R : nat → X → Y → Type
  n : nat
  a0 : X
  xs : list X
  xs' : list X
  l : list Y
  a : All2i R n ((a0 :: xs) ++ xs') l
  ============================
   (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,
        l = ys ++ ys'
        × All2i R n (a0 :: xs) ys × All2i R (n + #|a0 :: xs|) xs' ys')


DEBUG:Going to execute:
try simpl in H; DepElim.simplify_equations_in H
DEBUG LTAC Evaluated term: (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,
                                l = ys ++ ys'
                                × All2i R n (a0 :: xs) ys
                                  × All2i R (n + #|a0 :: xs|) xs' ys')