<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def gt_type(self):
 """The type of genotype, returns one of ``HOM_REF``, ``HOM_ALT``, and ``HET``. """

        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