code: def attributes(self, filter=Filter()):
          
          result = []
          for child, ancestry in self:
              if child.isattr() and child in filter:
                  result.append((child, ancestry))
          return result
docs: Get only the attribute content.
          @param filter: A filter to constrain the result.
          @type filter: L{Filter}
          @return: A list of tuples (attr, ancestry)
          @rtype: [(L{SchemaObject}, [L{SchemaObject},..]),..]