Add a utility function to check the first element in an sexp.

src/compare/util.rs

@@ -1,5 +1,7 @@
 use crate::parser::Source;
 
+use super::sexp::Token;
+
 /// Check if the child string slice is a slice of the parent string slice.
 fn is_slice_of(parent: &str, child: &str) -> bool {
     let parent_start = parent.as_ptr() as usize;
@@ -19,3 +21,19 @@ pub fn get_offsets<'s, S: Source<'s>>(source: &'s str, rust_object: &'s S) -> (u
     let end = offset + rust_object_source.len();
     (offset, end)
 }
+
+pub fn assert_name<'s>(emacs: &'s Token<'s>, name: &str) -> Result<(), Box<dyn std::error::Error>> {
+    let children = emacs.as_list()?;
+    let first_child = children
+        .first()
+        .ok_or("Should have at least one child.")?
+        .as_atom()?;
+    if first_child != name {
+        Err(format!(
+            "Expected a {expected} cell, but found a {found} cell.",
+            expected = name,
+            found = first_child
+        ))?;
+    }
+    Ok(())
+}