Function bracket_bound 

pub fn bracket_bound(axiom: &str, rules: &[(char, String)], depth: u32) -> usize

The most branch pushes the stream can hold open at once, for a grammar whose successors balance their brackets: the axiom’s own nesting, plus each of the depth expansion levels opening at most its deepest successor’s. An unbalanced grammar can open more; the turtle’s stack is sized to this and refuses a push past it.