diff --git a/src/main.rs b/src/main.rs
index e467b2f..720c13a 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -506,12 +506,12 @@ type Matches<'src> = HashMap<Symbol, &'src Sexp>;
 pub fn matches<'src>(vars: &[Symbol], lhs: &Sexp, expr: &'src Sexp) -> Option<Matches<'src>> {
     match (lhs, expr) {
         (Sexp::Atom(a), Sexp::Atom(b)) => {
-            if a == b {
-                Some(HashMap::with_capacity(0))
-            } else if vars.contains(a) {
+            if vars.contains(a) {
                 let mut out = HashMap::with_capacity(1);
                 out.insert(*a, expr);
                 Some(out)
+            } else if a == b {
+                Some(HashMap::with_capacity(0))
             } else {
                 None
             }
@@ -527,7 +527,7 @@ pub fn matches<'src>(vars: &[Symbol], lhs: &Sexp, expr: &'src Sexp) -> Option<Ma
         }
         (Sexp::List(_), Sexp::Atom(_)) => None,
         (Sexp::List(xs), Sexp::List(ys)) => {
-            if xs == ys {
+            if xs.is_empty() && ys.is_empty() {
                 Some(HashMap::with_capacity(0))
             } else if xs.len() == ys.len() {
                 xs.iter()
@@ -535,7 +535,7 @@ pub fn matches<'src>(vars: &[Symbol], lhs: &Sexp, expr: &'src Sexp) -> Option<Ma
                     .map(|(lhs, expr)| matches(vars, lhs, expr))
                     .reduce(|a, b| match (a, b) {
                         (None, _) | (_, None) => None,
-                        (Some(a), Some(b)) => merge_matches(vars, a, b),
+                        (Some(a), Some(b)) => merge_matches(vars, &a, &b),
                     })
                     .unwrap_or(None)
             } else {
@@ -545,10 +545,11 @@ pub fn matches<'src>(vars: &[Symbol], lhs: &Sexp, expr: &'src Sexp) -> Option<Ma
     }
 }
 
+#[must_use]
 pub fn merge_matches<'src>(
     vars: &[Symbol],
-    a: Matches<'src>,
-    b: Matches<'src>,
+    a: &Matches<'src>,
+    b: &Matches<'src>,
 ) -> Option<Matches<'src>> {
     let mut out = HashMap::with_capacity(vars.len());
 
@@ -579,6 +580,7 @@ pub fn merge_matches<'src>(
 //     }
 // }
 
+#[must_use]
 pub fn rw(rule: &Rule, sexp: &Sexp) -> Sexp {
     if rule.lhs() == rule.rhs() {
         return sexp.clone();