code: def is_purine(nucleotide, allow_extended_nucleotides=False):
      
      if not allow_extended_nucleotides and nucleotide not in STANDARD_NUCLEOTIDES:
          raise ValueError(
              "{} is a non-standard nucleotide, neither purine or pyrimidine".format(nucleotide))
      return nucleotide in PURINE_NUCLEOTIDES
docs: Is the nucleotide a purine