diff --git a/app-proto/src/assembly.rs b/app-proto/src/assembly.rs
index 1684716..07b0aba 100644
--- a/app-proto/src/assembly.rs
+++ b/app-proto/src/assembly.rs
@@ -119,6 +119,13 @@ pub struct Constraint {
     pub active: Signal<bool>
 }
 
+pub struct ElementMotion<'a> {
+    pub key: ElementKey,
+    pub velocity: DVectorView<'a, f64>
+}
+
+type AssemblyMotion<'a> = Vec<ElementMotion<'a>>;
+
 // a complete, view-independent description of an assembly
 #[derive(Clone)]
 pub struct Assembly {
@@ -290,7 +297,7 @@ impl Assembly {
     
     // --- deformation ---
     
-    pub fn deform(&self, element_motions: Vec<(ElementKey, DVectorView<f64>)>) {
+    pub fn deform(&self, motion: AssemblyMotion) {
         /* KLUDGE */
         // when the tangent space is zero, we currently need to avoid calling
         // its `proj` method, because it will panic rather than returning zero.
@@ -307,9 +314,9 @@ impl Assembly {
         
         // project the element motions onto the tangent space of the solution
         // variety, and sum them to get a deformation of the whole assembly
-        for (elt_key, elt_motion) in element_motions {
-            let column_index = self.elements.with(|elts| elts[elt_key].column_index);
-            motion_proj += self.tangent.with(|tan| tan.proj(&elt_motion, column_index));
+        for elt_motion in motion {
+            let column_index = self.elements.with(|elts| elts[elt_motion.key].column_index);
+            motion_proj += self.tangent.with(|tan| tan.proj(&elt_motion.velocity, column_index));
         }
         
         // step each element along the mass shell geodesic that matches its
diff --git a/app-proto/src/display.rs b/app-proto/src/display.rs
index 673985c..d8ee23c 100644
--- a/app-proto/src/display.rs
+++ b/app-proto/src/display.rs
@@ -14,7 +14,7 @@ use web_sys::{
     wasm_bindgen::{JsCast, JsValue}
 };
 
-use crate::{AppState, assembly::ElementKey};
+use crate::{AppState, assembly::{ElementKey, ElementMotion}};
 
 fn compile_shader(
     context: &WebGl2RenderingContext,
@@ -341,7 +341,14 @@ pub fn Display() -> View {
                         ])
                     };
                     let elt_motion: DVector<f64> = time_step * TRANSLATION_SPEED * vel_field * rep;
-                    assembly_for_raf.deform(vec![(sel, elt_motion.as_view())]);
+                    assembly_for_raf.deform(
+                        vec![
+                            ElementMotion {
+                                key: sel,
+                                velocity: elt_motion.as_view()
+                            }
+                        ]
+                    );
                     scene_changed.set(true);
                 }
             }