Change get_property to allow absent values.

We're returning an Option<> anyway so might as well handle absent values.

src/compare/util.rs

@@ -172,9 +172,7 @@ fn maybe_token_to_usize(
 
 /// Get a named property from the emacs token.
 ///
-/// Returns Ok(None) if value is nil.
+/// Returns Ok(None) if value is nil or absent.
-///
-/// Returns error if the attribute is not specified on the token at all.
 pub(crate) fn get_property<'b, 's, 'x>(
     emacs: &'b Token<'s>,
     key: &'x str,
@@ -185,14 +183,15 @@ pub(crate) fn get_property<'b, 's, 'x>(
         .nth(1)
         .ok_or("Should have an attributes child.")?;
     let attributes_map = attributes_child.as_map()?;
-    let prop = attributes_map
+    let prop = attributes_map.get(key).map(|token| *token);
-        .get(key)
+    match prop
-        .ok_or(format!("Missing {} attribute.", key))?;
+        .map(|token| token.as_atom())
-    match prop.as_atom() {
+        .map_or(Ok(None), |r| r.map(Some))?
-        Ok("nil") => return Ok(None),
+    {
+        Some("nil") => return Ok(None),
         _ => {}
     };
-    Ok(Some(*prop))
+    Ok(prop)
 }
 
 /// Get a named property containing an unquoted atom from the emacs token.