Goal:
  
  A : Type
  B : Type
  P : A → Type
  Q : A → B → Type
  H2 : ∀ (x : A) (y : B), In y [] → P x → Q x y
  Hl : 0 = 0
  Hl0 := {| pr1 := 0; pr2 := Hl |} : sigma (λ index : nat, 0 = index)
  ============================
   (let H := CoreTactics.block in let H0 := CoreTactics.block in All2 Q [] [])


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