docs: def transcript_to_gene(gtf):
      """
      return a dictionary keyed by transcript_id of the associated gene_id
      """
      gene_lookup = {}
      for feature in complete_features(get_gtf_db(gtf)):
          gene_id = feature.attributes.get('gene_id', [None])[0]
          transcript_id = feature.attributes.get('transcript_id', [None])[0]
          gene_lookup[transcript_id] = gene_id
      return gene_lookup