Function strip_leading_label 

pub(crate) fn strip_leading_label(
    src: &str,
    start: usize,
    end: usize,
) -> (usize, Option<ParsedLabel>)

If the substring src[start..end] begins with optional ASCII whitespace followed by <label>, return (label_body_start, Some(id)) where label_body_start is the offset just past the closing > (with any trailing whitespace also consumed). Otherwise return (start, None).

Only a single leading label is recognised; further <...> runs in the body are left intact for downstream stages.