Write the blob of each part in *parts* to the package, along with a
        rels item for its relationships if and only if it has any.

def _write_parts(phys_writer, parts):
        """
        Write the blob of each part in *parts* to the package, along with a
        rels item for its relationships if and only if it has any.
        """
        for part in parts:
            phys_writer.write(part.partname, part.blob)
            if len(part._rels):
                phys_writer.write(part.partname.rels_uri, part._rels.xml)