Goal:
  
  A : Type
  l3 : list A
  R1 : A → A → Type
  R2 : A → A → Type
  R3 : A → A → Type
  hd : A
  hd' : A
  tl : list A
  p : R1 hd hd'
  H1' : list A
  H1 : list A
  H : All2 R2 (pr1 {| pr1 := H1'; pr2 := H1 |})
        (pr2 {| pr1 := H1'; pr2 := H1 |})
  ============================
   ({| pr1 := hd :: tl; pr2 := l3 |} = {| pr1 := H1'; pr2 := H1 |}
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in
      (∀ x y : A, R2 x y → R3 x y)
      → (∀ x y z : A, R1 x y → R2 x z → R3 y z) → All2 R3 (hd' :: tl) l3)


DEBUG:Going to execute:
DepElim.decompose_exists xn id