def _uuid_for_part(model, part_name, is_rels = None):
    """Returns the uuid for the named part."""

    if part_name is None:
        return None
    if is_rels is None:
        is_rels = part_name.endswith('.rels')
    if is_rels:
        return model.rels_forest[part_name][0]
    return model.parts_forest[part_name][1]