Flutterby™! : Okay Rust geeks dumb newbie Q I need

Okay, Rust geeks, dumb newbie Q: I need a circular reference, so I'm going to just keep indices in a `Vec<Box<dyn ParseNode >>` arena.

The ParseNode trait has a `fn add_node(&mut self,...)` so that I can add the reference.

I want a `pub fn node(&self, idx : usize) -> &mut Box<dyn ParseNode>` to access that node, but I cannot figure out how to annotate the array to let me return the mutable reference that would let me call my `p.node(idx).add_node(other_idx)` method.