docs: def attributes(self):
          """
          A dictionary mapping names of attributes to BiomartAttribute instances.
  
          This causes overwriting errors if there are diffferent pages which use
          the same attribute names, but is kept for backward compatibility.
          """
          if not self._attribute_pages:
              self.fetch_attributes()
          result = {}
          for page in self._attribute_pages.values():
              result.update(page.attributes)
          return result