code: def is_snp(self):
          '''Returns true iff this variant is a SNP'''
          nucleotides = {'A', 'C', 'G', 'T'}
          return len(self.REF) == 1 and self.REF in nucleotides and set(self.ALT).issubset(nucleotidesf is_snp(self):
          '''Returns true iff this variant is a SNP'''
          nucleotides = {'A', 'C', 'G', 'T'}
          return len(self.REF) == 1 and self.REF in nucleotides and set(self.ALT).issubset(nucleotides)
docs: Returns true iff this variant is a SNP