<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def list_folder(cls, session, mailbox, folder):
 """Return conversations in a specific folder of a mailbox. Args: session (requests.sessions.Session):
 Authenticated session. mailbox (helpscout.models.Mailbox):
 Mailbox that folder is in. folder (helpscout.models.Folder):
 Folder to list. Returns: RequestPaginator(output_type=helpscout.models.Conversation):
 Conversations iterator. """

        return cls(
            '/mailboxes/%d/folders/%s/conversations.json' % (
                mailbox.id, folder.id,
            ),
            session=session,
        )