'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.'

@staticmethod
def _write_parts(phys_writer, parts):

    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)
