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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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