docs: def nary_subtree(self, tax_id, n=2):
          """Return a list of species tax_ids under *tax_id* such that
          node under *tax_id* and above the species has two children.
          """
          if tax_id is None:
              return None
          parent_id, rank = self._node(tax_id)
          if rank == 'species':
              return [tax_id]
          else:
              children = self.children_of(tax_id, 2)
              species_taxids = []
              for t in children:
                  species_taxids.extend(self.nary_subtree(t, n))
              return species_taxids