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