docs: def visit_member(self, attribute_key, attribute, member_node, member_data,
                       is_link_node, parent_data, index=None):
          """
          Visits a member node in a resource data tree.
  
          :param tuple attribute_key: tuple containing the attribute tokens
            identifying the member node's position in the resource data tree.
          :param attribute: mapped attribute holding information about the
            member node's name (in the parent) and type etc.
          :type attribute:
            :class:`everest.representers.attributes.MappedAttribute`
          :param member_node: the node holding resource data. This is either a
            resource instance (when using a :class:`ResourceTreeTraverser` on
            a tree of resources) or a data element instance (when using a
            :class:`DataElementTreeTraverser` on a data element tree.
          :param dict member_data: dictionary holding all member data
            extracted during traversal (with mapped attributes as keys).
          :param bool is_link_node: indicates if the given member node is a link.
          :param dict parent_data: dictionary holding all parent data extracted
            during traversal (with mapped attributes as keys).
          :param int index: this indicates a member node's index in a collection
            parent node. If the parent node is a member node, it will be `None`.
          """
          raise NotImplementedError('Abstract method.')