code: def gt_type(self):
          
          if not self.called:
              return None  # not called
          elif all(a == 0 for a in self.gt_alleles):
              return HOM_REF
          elif len(set(self.gt_alleles)) == 1:
              return HOM_ALT
          else:
              return HET
docs: The type of genotype, returns one of ``HOM_REF``, ``HOM_ALT``, and
          ``HET``.