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)
  a1 := pr1 a0 : list A
  a2 := pr2 a0 : (λ index : list A, All_fold P index) a1
  ============================
   (let H := CoreTactics.block in let H0 := CoreTactics.block in P [] x)


DEBUG:Going to execute:
hnf in (value of id'1), (value of id'2)
DEBUG LTAC Evaluated term: (pr1 a0)