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


DEBUG:Going to execute:
clear id' id