'Generate a 2-tuple `(source_uri, srel)` for each of the relationships
in the package.'

def iter_srels(self):

    for srel in self._pkg_srels:
        (yield (PACKAGE_URI, srel))
    for spart in self._sparts:
        for srel in spart.srels:
            (yield (spart.partname, srel))
