pub fn pre_order_visit_down<'a, T: 'a + Node>(
f: impl FnMut(&'a T) -> VortexResult<TraversalOrder>,
) -> impl NodeVisitor<'a, NodeTy = T>pub fn pre_order_visit_down<'a, T: 'a + Node>(
f: impl FnMut(&'a T) -> VortexResult<TraversalOrder>,
) -> impl NodeVisitor<'a, NodeTy = T>