use interior mutability for SourceFile lines

Using shared ownership for these may or may not have been a mistake in
the first place. We'll find out.

src/main.rs

@@ -35,10 +35,25 @@ fn main_wrapped() -> Result<(), Box<dyn StdError + Send + Sync + 'static>> {
         .custom_flags(libc::O_CLOEXEC);
     let source_file = SourceFile::open_from(Arc::clone(&def_path), opts)?;
 
-    let last_def_line = append_override::get_highest_prio(&args.name, source_file)?;
+    let last_def_line = append_override::get_highest_prio(&args.name, source_file.clone())?;
+    let pri: i64 = {
+        let text = last_def_line.text();
+        let pat = "lib.mkOverride (";
+        let start = text.find(pat).unwrap();
+        let substr = &text[start..];
+        let substr = substr.strip_prefix(pat).unwrap();
+        let end_paren = substr.find(")").unwrap();
+        let final_num = &substr[..end_paren];
+
+        final_num.parse().unwrap()
+    };
 
     eprintln!("{last_def_line}");
 
+    dbg!(&pri);
+
+    append_override::write_next_prio(source_file, last_def_line, pri, pri - 1)?;
+
     Ok(())
 }