Panic if we run out of serial numbers

app-proto/src/assembly.rs

@@ -44,13 +44,24 @@ 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");
+        
         Element {
             id: id,
             label: label,
             color: color,
             representation: create_signal(representation),
             constraints: create_signal(BTreeSet::default()),
-            serial: NEXT_ELEMENT_SERIAL.fetch_add(1, Ordering::SeqCst),
+            serial: serial,
             column_index: 0
         }
     }