def format_slack_mention(slack_id: str):
    """
    Formats a slack user ID as a mention using slack control sequences
    https://api.slack.com/docs/message-formatting#linking_to_channels_and_users

    >>> format_slack_mention('U5H9UR207')
    '<@U5H9UR207>'
    """
    return "<@{}>".format(slack_id)