Constant MAX_DOCUMENT_DEPTH 

pub const MAX_DOCUMENT_DEPTH: usize = 64;

Deepest element nesting a .reticle document may use before it is rejected.

Real documents nest three deep (reticle > elements > reticle-path > elements > reticle-path-line-to is five). The cap exists so a hostile document cannot grow the open-element stack without bound; nothing legitimate approaches it.