def get_d1nts(self):
        """Get letters for depth-01 GO terms, descendants count, and GO information."""
        data = []
        ntdata = cx.namedtuple("NtPrt", "D1 NS dcnt depth GO name")
        namespace = None
        for ntlet in sorted(self.goone2ntletter.values(),
                            key=lambda nt: [nt.goobj.namespace, -1 * nt.dcnt, nt.D1]):
            goobj = ntlet.goobj
            goid = goobj.id
            assert len(goobj.parents) == 1
            if namespace != goobj.namespace:
                namespace = goobj.namespace
                ntns = self.ns2nt[namespace]
                pobj = ntns.goobj
                ns2 = self.str2ns[goobj.namespace]
                data.append(ntdata._make([" ", ns2, ntns.dcnt, pobj.depth, pobj.id, pobj.name]))
            data.append(ntdata._make(
                [ntlet.D1, self.str2ns[namespace], ntlet.dcnt, goobj.depth, goid, goobj.name]))
        return data
Get letters for depth-01 GO terms, descendants count, and GO information.