code: def symbol_to_id(self, symbol):
          
  
          if symbol not in self.symbols_to_ids:
              m = 'Could not look up Entrez ID for Geneways symbol ' + symbol
              raise Exception(m)
          return self.symbols_to_ids[symbol]
docs: Returns the list of Entrez IDs for a given Geneways symbol
          (there may be more than one)