docs: def parse(self, func, *args, **kwargs):
          """ Parse element with given function"""
          result = []
          for element in self.xpath('child::node()'):
              if isinstance(element, Parser):
                  children = element.parse(func, *args, **kwargs)
                  element_result = func(element, children, *args, **kwargs)
                  if element_result:
                      result.append(element_result)
              else:
                  result.append(element)
          return u"".join(result)