code: def gene_ids_of_gene_name(self, gene_name):
          
          results = self._query_gene_ids("gene_name", gene_name)
          if len(results) == 0:
              raise ValueError("Gene name not found: %s" % gene_name)
          return results
docs: What are the gene IDs associated with a given gene name?
          (due to copy events, there might be multiple genes per name)