Function parse_with 

pub fn parse_with(source: &str, marks: &Marks) -> Doc

parse with the app’s own marks — see crate::Marks.

Registered delimiters are lifted out of the source before CommonMark sees it, which is the only place the difference between == and \=\= still exists: a backslash escape is gone by the time there is a Text to scan, and a pass over one would read an escaped delimiter back as a mark and move the document on every save.