Goal:
  
  A : Type
  R1 : A → A → Type
  R2 : A → Type
  R3 : A → Type
  hd : A
  tl : list A
  tl' : list A
  o : OnOne2 R1 tl tl'
  IHo : All R2 tl
        → (∀ x : A, R2 x → R3 x)
          → (∀ x y : A, R1 x y → R2 x → R3 y) → All R3 tl'
  H : All R2 (hd :: tl)
  H0 := {| pr1 := hd :: tl; pr2 := H |}
     : sigma (λ index : list A, All R2 index)
  ============================
   (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:
set (id'2 := pr2 id'); set (id'1 := pr1 id') in id'2;
            hnf in (value of id'1), (value of id'2)