Extract the line number from the switches.

src/types/lesser_element.rs

@@ -108,7 +108,10 @@ pub struct LatexEnvironment<'s> {
     pub source: &'s str,
 }
 
-pub type LineNumber = usize;
+/// A line number used in switches to lesser blocks.
+///
+/// This must be signed because emacs subtracts 1 from the actual value in the org-mode text, which makes a 0 turn into a -1.
+pub type LineNumber = isize;
 
 #[derive(Debug)]
 pub enum SwitchNumberLines {