docs: def stringClade(taxrefs, name, at):
      '''Return a Newick string from a list of TaxRefs'''
      string = []
      for ref in taxrefs:
          # distance is the difference between the taxonomic level of the ref
          #  and the current level of the tree growth
          d = float(at-ref.level)
          # ensure no spaces in ident, Newick tree cannot have spaces
          ident = re.sub("\s", "_", ref.ident)
          string.append('{0}:{1}'.format(ident, d))
      # join into single string with a name for the clade
      string = ','.join(string)
      string = '({0}){1}'.format(string, name)
      return string