Goal:
    A : Type  P : list A → A → Type  x : A  a : All_fold P [x]
  ============================
   (let H := CoreTactics.block in P [] x)


DEBUG:Going to execute:
fun id' =>
             match id' with
             | id => idtac
             | _ => revert id'; CoreTactics.revert_until id
             end