docs: def _set_variant_id(self, variant_id=None):
          """Set the variant id for this variant"""
          if not variant_id:
              variant_id = '_'.join([
                  self.CHROM,
                  str(self.POS),
                  self.REF,
                  self.ALT
                  ])
  
          logger.debug("Updating variant id to {0}".format(
              variant_id))
  
          self['variant_id'] = variant_id