code: def add_variant_args(arg_parser):
      
      variant_arg_group = arg_parser.add_argument_group(
          title="Variants",
          description="Genomic variant files")
  
      variant_arg_group.add_argument(
          "--vcf",
          default=[],
          action="append",
          help="Genomic variants in VCF format")
  
      variant_arg_group.add_argument(
          "--maf",
          default=[],
          action="append",
          help="Genomic variants in TCGA's MAF format",)
  
      variant_arg_group.add_argument(
          "--variant",
          default=[],
          action="append",
          nargs=4,
          metavar=("CHR", "POS", "REF", "ALT"),
          help=(
              "Individual variant as 4 arguments giving chromsome, position, ref,"
              " and alt. Example: chr1 3848 C G. Use '.' to indicate empty alleles"
              " for insertions or deletions."))
  
      variant_arg_group.add_argument(
          "--genome",
          type=str,
          help=(
              "What reference assembly your variant coordinates are using. "
              "Examples: 'hg19', 'GRCh38', or 'mm9'. "
              "This argument is ignored for MAF files, since each row includes "
              "the reference. "
              "For VCF files, this is used if specified, and otherwise is guessed from "
              "the header. For variants specfied on the commandline with --variant, "
              "this option is required."))
  
      variant_arg_group.add_argument(
          "--download-reference-genome-data",
          action="store_true",
          default=False,
          help=(
              ("Automatically download genome reference data required for "
               "annotation using PyEnsembl. Otherwise you must first run "
               "'pyensembl install' for the release/species corresponding "
               "to the genome used in your VCF.")))
  
      variant_arg_group.add_argument(
          "--json-variants",
          default=[],
          action="append",
          help="Path to Varcode.VariantCollection object serialized as a JSON file.")
  
      return variant_arg_group
docs: Extends an ArgumentParser instance with the following commandline arguments:
          --vcf
          --genome
          --maf
          --variant
          --json-variants