site.alleles() method?

[hyanwong]

I just realised that I can't get the alleles defined at a site. I'd prefer not to use the variants() method to do this. Is there any reason why we can't define a helper function:

class Site:
    def alleles(self):
        alleles = set((m.derived_state for m in self.mutations))
        alleles.add(self.ancestral_state)
        return alleles

[jeromekelleher]

Seems reasonable.