def _create_ref(ref_type, pkg_name, name):
    """Creates a reference for the specified type.

    Args:
        ref_type: A `string` value. Must be one of the values in
                  `reference_types`.
        pkg_name: The package name as a `string`.
        name: The name of the item as a `string`.

    Returns:
        A `string` that references an entity in a set of package descriptions.
    """
    return "%s:%s/%s" % (ref_type, pkg_name, name)