pub(crate) fn new() -> Self {

                Self {

                    $(

                        [<before_ $dir>]: vec![],

                        [<after_ $dir>]: vec![],

                    )*

                }

            }

                pub(crate) fn [<trigger_before_ $dir>](&self, node: &T, meta: &mut M) {

                    for f in &self.[<before_ $dir>] {

                        f(node, meta)

                }

                pub(crate) fn [<trigger_after_ $dir>](&self, node: &T, meta: &mut M) {

                    for f in &self.[<after_ $dir>] {

                        f(node, meta)

                }

                pub(crate) fn [<add_before_ $dir>](&mut self, handler: fn(&T, &mut M)) {

                    self.[<before_ $dir>].push(handler);

                }

                pub(crate) fn [<add_after_ $dir>](&mut self, handler: fn(&T, &mut M)) {

                    self.[<after_ $dir>].push(handler);

                }