def primary_mrna(entrystream, parenttype='gene'):
    for entry in entrystream:
        if not isinstance(entry, tag.Feature):
            yield entry
            continue
        for parent in tag.select.features(entry, parenttype, traverse=True):
            mrnas = [f for f in parent.children if f.type == 'mRNA']
            if len(mrnas) == 0:
                continue
            _emplace_pmrna(mrnas, parent)
        yield entry