def content_types(content_type):
    """Return an ordered tuple of content types from pyxst `textual_content_type` that may be None, a
    set or a string value.
    """
    if content_type:
        if isinstance(content_type, set):
            content_types = sorted(content_type)
        else:
            if content_type == 'IDREF':
                content_type = 'NCName'
            content_types = content_type,
    else:
        content_types = ()
    return content_types
