Function remove_line 

pub fn remove_line(
    path: &Path,
    place: usize,
    seen: &SeenLine,
) -> Result<bool, String>

Takes one line out of the file: the one at place among the lines (as sioul:budget/<budget>/<place> names it) when it is still the line seen, else the one line that is (find_line). Ok(false) when the line is not found as it was: nothing is changed, for the page to say so.