diff --git a/src/mem/basic.rs b/src/mem/basic.rs
index dcffafe..9838511 100644
--- a/src/mem/basic.rs
+++ b/src/mem/basic.rs
@@ -45,6 +45,7 @@ impl Gc for BasicGc {
     }
 }
 
+#[derive(Default)]
 pub struct BasicIntern {
     strs: HashMap<String, ObjRef<Str>>,
     syms: HashMap<String, Sym>,
diff --git a/src/mem/mod.rs b/src/mem/mod.rs
index f2d9f88..f6e6378 100644
--- a/src/mem/mod.rs
+++ b/src/mem/mod.rs
@@ -3,11 +3,12 @@ pub mod gc;
 pub mod intern;
 pub mod ptr;
 
-pub use basic::BasicGc;
+pub use basic::{BasicGc, BasicIntern};
 
 pub mod prelude {
     pub use crate::mem::{
         BasicGc,
+        BasicIntern,
         gc::Gc,
         intern::Intern,
         ptr::{DynRef, ObjRef},
diff --git a/src/obj/attrs.rs b/src/obj/attrs.rs
index 0544c1c..af7aaa5 100644
--- a/src/obj/attrs.rs
+++ b/src/obj/attrs.rs
@@ -121,15 +121,16 @@ impl<'i, I: Intern> AttrsBuilder<'i, I> {
 
 #[test]
 fn test_attrs_new() {
-    use crate::mem::BasicGc;
+    use crate::mem::{BasicGc, BasicIntern};
 
     let mut gc = BasicGc::default();
+    let mut intern = BasicIntern::default();
     
     let attrs_ref = Attrs::new(&mut gc, Default::default());
 
     {
         let mut attrs = attrs_ref.borrow_mut();
-        let sym = gc.add_sym("symbol");
+        let sym = intern.intern_sym("symbol");
         attrs.insert(sym, attrs_ref.as_dyn());
 
         assert!(
diff --git a/src/obj/str.rs b/src/obj/str.rs
index bcf5a19..9b0979d 100644
--- a/src/obj/str.rs
+++ b/src/obj/str.rs
@@ -44,7 +44,7 @@ impl Obj for Str {
             return;
         }
         self.marked.set(true);
-        todo!()
+        self.attrs.borrow().mark();
     }
 
     fn unmark(&self) {