diff --git a/app-proto/src/assembly.rs b/app-proto/src/assembly.rs
index 8cf3bb3..bfbb2b8 100644
--- a/app-proto/src/assembly.rs
+++ b/app-proto/src/assembly.rs
@@ -1,7 +1,7 @@
 use nalgebra::{DMatrix, DVector};
 use rustc_hash::FxHashMap;
 use slab::Slab;
-use std::{collections::BTreeSet, sync::atomic::{AtomicU64, Ordering}};
+use std::{cell::Cell, collections::BTreeSet};
 use sycamore::prelude::*;
 use web_sys::{console, wasm_bindgen::JsValue}; /* DEBUG */
 
@@ -13,12 +13,9 @@ pub type ConstraintKey = usize;
 
 pub type ElementColor = [f32; 3];
 
-/* KLUDGE */
-// we should reconsider this design when we build a system for switching between
-// assemblies. at that point, we might want to switch to hierarchical keys,
-// where each each element has a key that identifies it within its assembly and
-// each assembly has a key that identifies it within the sesssion
-static NEXT_ELEMENT_SERIAL: AtomicU64 = AtomicU64::new(0);
+thread_local! {
+    static NEXT_ELEMENT_SERIAL: Cell<u64> = Cell::new(0);
+}
 
 #[derive(Clone, PartialEq)]
 pub struct Element {
@@ -44,16 +41,9 @@ impl Element {
         color: ElementColor,
         representation: DVector<f64>
     ) -> Element {
-        // take the next serial number, panicking if that was the last number we
-        // had left. the technique we use to panic on overflow is taken from
-        // _Rust Atomics and Locks_, by Mara Bos
-        //
-        //   https://marabos.nl/atomics/atomics.html#example-handle-overflow
-        //
-        let serial = NEXT_ELEMENT_SERIAL.fetch_update(
-            Ordering::SeqCst, Ordering::SeqCst,
-            |serial| serial.checked_add(1)
-        ).expect("Out of serial numbers for elements");
+        // take the next serial number
+        let serial = NEXT_ELEMENT_SERIAL.get();
+        NEXT_ELEMENT_SERIAL.set(serial.wrapping_add(1));
         
         Element {
             id: id,