code: def is_snp(reference_bases, alternate_bases):
      
      if len(reference_bases) > 1:
          return False
      for alt in alternate_bases:
          if alt is None:
              return False
          if alt not in ['A', 'C', 'G', 'T', 'N', '*']:
              return False
      return True
docs: Return whether or not the variant is a SNP