code: def domain_of_validity(self):
          
          # TODO: Generalise interface to return a polygon? (Can we find
          # something that uses a polygon instead?)
          domain = self.element.find(GML_NS + 'domainOfValidity')
          domain_href = domain.attrib[XLINK_NS + 'href']
          url = '{prefix}{code}.gml?download'.format(prefix=EPSG_IO_URL,
                                                     code=domain_href)
          xml = requests.get(url).content
          gml = ET.fromstring(xml)
  
          def extract_bound(tag):
              ns = '{http://www.isotc211.org/2005/gmd}'
              xpath = './/{ns}EX_GeographicBoundingBox/{ns}{tag}/'.format(
                  ns=ns,
                  tag=tag)
              bound = gml.find(xpath)
              return float(bound.text)
  
          tags = ('westBoundLongitude', 'eastBoundLongitude',
                  'southBoundLatitude', 'northBoundLatitude')
          bounds = [extract_bound(tag) for tag in tags]
          return bounds
docs: Return the domain of validity for this CRS as:
          (west, east, south, north).
  
          For example::
  
              >>> print(get(21781).domain_of_validity())
              [5.96, 10.49, 45.82, 47.81]