'Determine all possible recipients of Inboxes for this object.
`obj` may be one of (Comment, Message).'

@classmethod
def possible_recipients(cls, obj):

    possible_recipients = []
    if isinstance(obj, Comment):
        parent_id = getattr(obj, 'parent_id', None)
        if parent_id:
            parent_comment = Comment._byID(parent_id, data=True)
            possible_recipients.append(parent_comment.author_id)
        else:
            parent_link = Link._byID(obj.link_id, data=True)
            possible_recipients.append(parent_link.author_id)
        mentions = utils.extract_user_mentions(obj.body)
        if (len(mentions) <= g.butler_max_mentions):
            possible_recipients.extend(Account._names_to_ids(mentions, ignore_missing=True))
    elif isinstance(obj, Message):
        if obj.to_id:
            possible_recipients.append(obj.to_id)
    else:
        g.log.warning('Unknown    object    type    for    recipients:    %r', obj)
    return possible_recipients
