docs: def _variants_arities(fields, arities, info_counts):
      """Utility function to determine arities (i.e., number of values to
      expect) for variants fields."""
      if arities is None:
          # no arities specified by user
          arities = dict()
      for f, vcf_count in zip(fields, info_counts):
          if f == 'FILTER':
              arities[f] = 1  # force one value for the FILTER field
          elif f not in arities:
              # arity not specified by user
              if f in config.STANDARD_VARIANT_FIELDS:
                  arities[f] = config.DEFAULT_VARIANT_ARITY[f]
              elif vcf_count == NUMBER_ALLELE:
                  # default to 1 (biallelic)
                  arities[f] = 1
              elif vcf_count <= 0:
                  # catch any other cases of non-specific arity
                  arities[f] = 1
              else:
                  # use arity (i.e., number) specified in INFO header
                  arities[f] = vcf_count
      # convert to tuple for zipping with fields
      arities = tuple(arities[f] for f in fields)
      return arities