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(())
+}