code: def get_gene_source_set(gtf):
      
      gene_to_source = {}
      db = get_gtf_db(gtf)
      for feature in complete_features(db):
          gene_id = feature['gene_id'][0]
          sources = gene_to_source.get(gene_id, set([])).union(set([feature.source]))
          gene_to_source[gene_id] = sources
      return gene_to_source
docs: get a dictionary of the set of all sources for a gene