docs: def rna_transcript_expression_dict_from_args(args):
      """
      Returns a dictionary mapping Ensembl transcript IDs to FPKM expression
      values or None if neither Cufflinks tracking file nor StringTie GTF file
      were specified.
      """
      if args.rna_transcript_fpkm_tracking_file:
          return load_cufflinks_fpkm_dict(args.rna_transcript_fpkm_tracking_file)
      elif args.rna_transcript_fpkm_gtf_file:
          return load_transcript_fpkm_dict_from_gtf(
              args.rna_transcript_fpkm_gtf_file)
      else:
          return None