diff --git a/src/protocol/arena.rs b/src/protocol/arena.rs
index 2120f5d9b983c1af93ba6530e5a6565e15d101eb..86876643d3dd9c922c2fb748de5cc52403422e2f 100644
--- a/src/protocol/arena.rs
+++ b/src/protocol/arena.rs
@@ -52,6 +52,9 @@ impl<T> Arena<T> {
     pub fn iter(&self) -> impl Iterator<Item = &T> {
         self.store.iter()
     }
+    pub fn len(&self) -> usize {
+        self.store.len()
+    }
 }
 impl<T> core::ops::Index<Id<T>> for Arena<T> {
     type Output = T;