code: def get_transcript_source_set(gtf):
      
      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
docs: get a dictionary of the set of all sources of the gene for a given
      transcript