Function math_source 

pub fn math_source(tex: &str, display: bool) -> String

Reconstruct an equation’s markdown source — the TeX with its $ / $$ delimiters restored — from the inner TeX pulldown hands the walker.

pub because plain-text collectors are not confined to this crate: the email walker in moss::infra::newsletter builds an image alt attribute and needs the same restored-delimiter form, or the two surfaces disagree about what an equation looks like in plain text.