(* This marker is used in combination with a notation (see the end of the file),
   in order to hide the exact expression of a subgoal, and to indicate that
   faced with this subgoal, the user always has to call the
   [end defer] tactic. *)
Definition end_defer (P : Type) := P.(* This marker is used to keep track of "group" assumptions in the context
   (introduced by [begin procrastination]). This is useful so that tactics can
   omit specifying a group: in that case, the first group of the context is
   used. *)
Definition group (P : Prop) := P.

Lemma group_fold : forall (P: Prop), P -> group P.