<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def pop(self):
 """Pop the top revision off the stack back onto the collection at the given id. This method applies the action. Note: This assumes you don't have two revisions scheduled closer than a single scheduling cycle. """

        revisions = yield self.list()

        if len(revisions) > 0:
            revision = revisions[0]

            # Update type action
            if revision.get("action") == self.UPDATE_ACTION:
                try:
                    yield self.__update_action(revision)
                except Exception as ex:
                    self.logger.error(ex)

            # Insert type update
            if revision.get("action") == self.INSERT_ACTION:
                try:
                    yield self.__insert_action(revision)
                except Exception as ex:
                    self.logger.error(ex)

            #Get the updated object for attachment to the snapshot
            snapshot_object = yield self.collection.find_one_by_id(revision.get("master_id"))

            #Handle delete action here
            if revision.get("action") == self.DELETE_ACTION:
                try:
                    yield self.__delete_action(revision)
                except Exception as ex:
                    self.logger.error(ex)

                snapshot_object = None

            #Update the revision to be in a post-process state including snapshot
            revision_update_response = yield self.revisions.patch(revision.get("id"),
                {
                    "processed" : True,
                    "snapshot" : snapshot_object,
                    "inProcess": False
                }
            )

            if revision_update_response.get("n") == 0:
                raise RevisionUpdateFailed(msg="revision document update failed")

            revision = yield self.revisions.find_one_by_id(revision.get("id"))

            #TODO: Make this callback method something that can be passed in.  This was used in
            #the original implementation to send back to the client via websocket
            #revision_success.send('revision_success', type="RevisionSuccess", data=revision)

            raise Return(revision)

        raise Return(None)