docs: def _small_mrca(self, ts):
          """Find a MRCA using query parameters.
  
          This only supports a limited number of tax_ids; ``_large_mrca`` will
          support an arbitrary number.
          """
          cursor = self.db.cursor()
          qmarks = ', '.join('?' * len(ts))
          cursor.execute("""
              SELECT parent
              FROM   parents
                     JOIN taxa
                       ON parent = taxa.tax_id
                     JOIN ranks USING (rank)
              WHERE  child IN (%s)
              GROUP  BY parent
              HAVING COUNT(*) = ?
              ORDER  BY rank_order DESC
              LIMIT  1
          """ % qmarks, ts + (len(ts),))
  
          return cursor.fetchall()