Move get_property into util.

src/compare/util.rs

@@ -137,3 +137,23 @@ fn maybe_token_to_usize(
         .flatten() // Outer option is whether or not the param exists, inner option is whether or not it is nil
         .map_or(Ok(None), |r| r.map(Some))?)
 }
+
+pub fn get_property<'s, 'x>(
+    emacs: &'s Token<'s>,
+    key: &'x str,
+) -> Result<Option<&'s Token<'s>>, Box<dyn std::error::Error>> {
+    let children = emacs.as_list()?;
+    let attributes_child = children
+        .iter()
+        .nth(1)
+        .ok_or("Should have an attributes child.")?;
+    let attributes_map = attributes_child.as_map()?;
+    let prop = attributes_map
+        .get(key)
+        .ok_or(format!("Missing {} attribute.", key))?;
+    match prop.as_atom() {
+        Ok("nil") => return Ok(None),
+        _ => {}
+    };
+    Ok(Some(*prop))
+}