get a dictionary of the set of all sources of the gene for a given
    transcript

def get_transcript_source_set(gtf):
    """
    get a dictionary of the set of all sources of the gene for a given
    transcript
    """
    gene_to_source = get_gene_source_set(gtf)
    transcript_to_source = {}
    db = get_gtf_db(gtf)
    for feature in complete_features(db):
        gene_id = feature['gene_id'][0]
        transcript_to_source[feature['transcript_id'][0]] = gene_to_source[gene_id]
    return transcript_to_source