code: def variant_indices(self):
          
          variant_start_index = len(self.prefix)
          variant_len = len(self.alt)
          variant_end_index = variant_start_index + variant_len
          return variant_start_index, variant_end_index
docs: When we combine prefix + alt + suffix into a single string,
          what are is base-0 index interval which gets us back the alt
          sequence? First returned index is inclusive, the second is exclusive.