code: def ncbi_geneid(self):
          
          values = self.get_attribute('Dbxref', as_list=True)
          if values is None:
              return None
          for value in values:
              if value.startswith('GeneID:'):
                  key, geneid = value.split(':')
                  return geneid
          return None
docs: Retrieve this feature's NCBI GeneID if it's present.
  
          NCBI GFF3 files contain gene IDs encoded in **Dbxref** attributes
          (example: `Dbxref=GeneID:103504972`). This function locates and returns
          the GeneID if present, or returns `None` otherwise.