docs: def _variants_fills(fields, fills, info_types):
      """Utility function to determine fill values for variants fields with
      missing values."""
      if fills is None:
          # no fills specified by user
          fills = dict()
      for f, vcf_type in zip(fields, info_types):
          if f == 'FILTER':
              fills[f] = False
          elif f not in fills:
              if f in config.STANDARD_VARIANT_FIELDS:
                  fills[f] = config.DEFAULT_VARIANT_FILL[f]
              else:
                  fills[f] = config.DEFAULT_FILL_MAP[vcf_type]
      # convert to tuple for zipping with fields
      fills = tuple(fills[f] for f in fields)
      return fills