'Given a comment, set it as the sticky (top) comment for this link.
Only one comment may be stickied at a time, and stickied comments must
be top level. `set_by` is an optional Account, which if set will add
a ModAction event to the mod log.
Raises `RedditError` on an attempt to sticky non-top-level comments.'

def set_sticky_comment(self, comment, set_by=None):

    if (not comment.is_stickyable):
        raise RedditError('COMMENT_NOT_STICKYABLE', code=400)
    if (self.sticky_comment_id == comment._id):
        return
    self.remove_sticky_comment(set_by=set_by)
    self.sticky_comment_id = comment._id
    self._commit()
    if set_by:
        ModAction.create(self.subreddit_slow, set_by, action='sticky', target=comment)
