docs: def tx2genefile(gtf, out_file=None):
      """
      write out a file of transcript->gene mappings.
      use the installed tx2gene.csv if it exists, else write a new one out
      """
      installed_tx2gene = os.path.join(os.path.dirname(gtf), "tx2gene.csv")
      if file_exists(installed_tx2gene):
          return installed_tx2gene
      if file_exists(out_file):
          return out_file
      with file_transaction(out_file) as tx_out_file:
          with open(tx_out_file, "w") as out_handle:
              for k, v in transcript_to_gene(gtf).items():
                  out_handle.write(",".join([k, v]) + "\n")
      return out_file