diff --git a/sk_demo/src/Rig.cs b/sk_demo/src/Rig.cs
index 2f4c7f0..dbc377b 100644
--- a/sk_demo/src/Rig.cs
+++ b/sk_demo/src/Rig.cs
@@ -30,4 +30,18 @@ public class Rig {
 
     perch_pos = palm_pos + Vec3.Up * 8 * U.cm;
   }
+
+  public struct Btn {
+    public bool frameDown, held, frameUp;
+
+    public void Frame(bool down, bool? up = null) {
+      if (up != null && held) {
+        down = !(bool)up;
+      }
+
+      frameDown = down && !held;
+      frameUp = !down && held;
+      held = down;
+    }
+  }
 }
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