Goal:
  
  A : Type
  P : list A → A → Type
  x : A
  a : All_fold P [x]
  a0 := {| pr1 := [x]; pr2 := a |}
     : sigma (λ index : list A, All_fold P index)
  ============================
   (let H := CoreTactics.block in let H0 := CoreTactics.block in P [] x)


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)