code: def _variants_fields(fields, exclude_fields, info_ids):
      
      if fields is None:
          # no fields specified by user
          # by default extract all standard and INFO fields
          fields = config.STANDARD_VARIANT_FIELDS + info_ids
      else:
          # fields have been specified
          for f in fields:
              # check for non-standard fields not declared in INFO header
              if f not in config.STANDARD_VARIANT_FIELDS and f not in info_ids:
                  # support extracting INFO even if not declared in header,
                  # but warn...
                  print('WARNING: no INFO definition found for field %s' % f,
                        file=sys.stderr)
      # process any exclusions
      if exclude_fields is not None:
          fields = [f for f in fields if f not in exclude_fields]
      return tuple(f for f in fields)
docs: Utility function to determine which fields to extract when loading
      variants.