fn discover<'a>(
rest: &'a str,
start_fence: Option<&'a str>,
markdown: bool,
lines: &mut Vec<(usize, Option<&'a str>)>,
upto: usize,
)Expand description
Extends lines to cover at least upto lines of rest, or until rest
is exhausted. Each entry pairs the byte offset past a line with the fence
info open after it (markdown mode only).