Goal:
  
  A : Type
  R1 : A → A → Type
  R2 : A → Type
  R3 : A → Type
  hd : A
  hd' : A
  tl : list A
  p : R1 hd hd'
  H : All R2 (hd :: tl)
  H0 := {| pr1 := hd :: tl; pr2 := H |}
     : sigma (λ index : list A, All R2 index)
  H1 := hd :: tl : list A
  H2 := H : (λ index : list A, All R2 index) H1
  ============================
   (H = H2
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in
      (∀ x : A, R2 x → R3 x)
      → (∀ x y : A, R1 x y → R2 x → R3 y) → All R3 (hd' :: tl))


DEBUG:Going to execute:
<coq-core.plugins.ltac::clearbody@0> $1