diff --git a/snake_egg/tests/test_egg_interface.py b/snake_egg/tests/test_egg_interface.py
index 1399cf7..63d12f7 100644
--- a/snake_egg/tests/test_egg_interface.py
+++ b/snake_egg/tests/test_egg_interface.py
@@ -14,10 +14,15 @@ def __init__(self, x: Any, y: Any):
         self.x = x
         self.y = y
 
+    def __str__(self) -> str:
+        return f"Add({self.x}, {self.y})"
+
+    @property
     def __egg_head__(self):
         return self.__class__
 
-    def __egg_args(self):
+    @property
+    def __egg_args__(self):
         return self.x, self.y
 
 
@@ -26,10 +31,15 @@ def __init__(self, x: Any, y: Any):
         self.x = x
         self.y = y
 
+    def __str__(self) -> str:
+        return f"Mul({self.x}, {self.y})"
+
+    @property
     def __egg_head__(self):
         return self.__class__
 
-    def __egg_args(self):
+    @property
+    def __egg_args__(self):
         return self.x, self.y
 
 
@@ -45,25 +55,29 @@ def __egg_args(self):
 ]
 
 
-def simplify(expr, iters=7):
+def is_equal(expr_a, expr_b, iters=5):
     egraph = EGraph()
-    egraph.add(expr)
+
+    id_a = egraph.add(expr_a)
+    id_b = egraph.add(expr_b)
+
     egraph.run(rules, iters)
-    best = egraph.extract(expr)
-    return best
+
+    return egraph.equiv(id_a, id_b)
 
 
 def test_simple_1():
-    assert simplify(Mul(0, 42)) == 0
+    assert is_equal(Mul(0, 42), 0)
 
 
 def test_simple_2():
     foo = "foo"
-    assert simplify(Add(0, Mul(1, foo))) == foo
+    assert is_equal(Add(0, Mul(1, foo)), foo)
 
 
 def test_simple_3():
-    assert simplify(Mul(2, Mul(1, "foo"))) == Mul(2, "foo")
+    foo = "foo"
+    assert is_equal(Mul(2, Mul(1, foo)), Mul(2, foo))
 
 
 test_simple_1()
diff --git a/src/util.rs b/src/util.rs
index 18edd78..62b329c 100644
--- a/src/util.rs
+++ b/src/util.rs
@@ -17,7 +17,7 @@ pub fn build_node(egraph: &mut EGraph<PythonNode, PythonAnalysis>, expr: &PyAny)
         egraph.find(id)
     } else if let Ok(PyVar(var)) = expr.extract() {
         panic!("Can't add a var: {}", var)
-    } else if let Ok(args) = expr.getattr("__egg_args") {
+    } else if let Ok(args) = expr.getattr("__egg_args__") {
 	let args = args.downcast::<PyTuple>().unwrap();
 	let class = if let Ok(class) = expr.getattr("__egg_head__") {
             class.downcast::<PyType>().unwrap()