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`.

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.')