Goal:
  
  A : Type
  B : Type
  P : A → B → Type
  Q : A → B → Type
  H : All2 Q [] []
  H0 := {| pr1 := {| pr1 := []; pr2 := [] |}; pr2 := H |}
     : sigma
         (λ index : sigma (λ _ : list A, list B),
            All2 Q (pr1 index) (pr2 index))
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in All2 (Trel_conj P Q) [] [])


DEBUG:Going to execute:
set (id'2 := pr2 id'); set (id'1 := pr1 id') in id'2