def add(self, elem, replace=False):
        """Add a requirement.

        :param elem: a string or :class:`Requirement` instance.
        :param replace: if `True`, packages in the set with the same name will
            be removed first.
        """
        if isinstance(elem, text_type):
            elem = Requirement.coerce(elem)
        if replace and elem in self:
            self.remove(elem)
        super(Requirements, self).add(elem)
def db_version_control(engine, abs_path, version=None):
    """Mark a database as under this repository's version control.

    Once a database is under version control, schema changes should
    only be done via change scripts in this repository.

    :param engine:  SQLAlchemy engine instance for a given database
    :param abs_path: Absolute path to migrate repository
    :param version:  Initial database version
    """
    repository = _find_migrate_repo(abs_path)
    versioning_api.version_control(engine, repository, version)
    return version