Just barely starting to diff the two parsed forms.

src/compare/diff.rs

@@ -0,0 +1,25 @@
+use super::sexp::Token;
+use crate::parser::Document;
+
+pub fn compare_document<'s>(
+    emacs: &'s Token<'s>,
+    rust: &'s Document<'s>,
+) -> Result<(), Box<dyn std::error::Error>> {
+    compare_document_indented(0, emacs, rust)
+}
+
+fn compare_document_indented<'s>(
+    indentation: usize,
+    emacs: &'s Token<'s>,
+    rust: &'s Document<'s>,
+) -> Result<(), Box<dyn std::error::Error>> {
+    let children = emacs.as_list()?;
+    let first_child = children.first().ok_or("Should have at least one child")?;
+    let first_child_text = first_child.as_atom()?;
+    if first_child_text != "org-data" {
+        return Err("Document should correspond to an org-data cell.".into());
+    }
+    // TODO: compare the children
+
+    Ok(())
+}

src/compare/mod.rs

@@ -1,5 +1,7 @@
+mod diff;
 mod error;
 mod parse;
 mod sexp;
+pub use diff::compare_document;
 pub use parse::emacs_parse_org_document;
 pub use sexp::sexp;

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)?;