Just barely starting to diff the two parsed forms.

src/compare/sexp.rs

@@ -30,6 +30,22 @@ pub struct TextWithProperties<'s> {
     properties: Vec<Token<'s>>,
 }
 
+impl<'s> Token<'s> {
+    pub fn as_list<'p>(&'p self) -> Result<&'p Vec<Token<'s>>, Box<dyn std::error::Error>> {
+        Ok(match self {
+            Token::List(children) => Ok(children),
+            _ => Err("wrong token type"),
+        }?)
+    }
+
+    pub fn as_atom<'p>(&'p self) -> Result<&'s str, Box<dyn std::error::Error>> {
+        Ok(match self {
+            Token::Atom(body) => Ok(*body),
+            _ => Err("wrong token type"),
+        }?)
+    }
+}
+
 #[tracing::instrument(ret, level = "debug")]
 pub fn sexp<'s>(input: &'s str) -> Res<&'s str, Token<'s>> {
     let (remaining, _) = multispace0(input)?;