diff --git a/app-proto/full-interface/src/assembly.rs b/app-proto/full-interface/src/assembly.rs
index aa4355e..0660d0e 100644
--- a/app-proto/full-interface/src/assembly.rs
+++ b/app-proto/full-interface/src/assembly.rs
@@ -39,6 +39,25 @@ impl Assembly {
         }
     }
     
+    // insert an element into the assembly without checking whether we already
+    // have an element with the same identifier. any element that does have the
+    // same identifier will get kicked out of the `elements_by_id` index
+    fn insert_element_unchecked(&self, elt: Element) {
+        let id = elt.id.clone();
+        let key = self.elements.update(|elts| elts.insert(elt));
+        self.elements_by_id.update(|elts_by_id| elts_by_id.insert(id, key));
+    }
+    
+    pub fn try_insert_element(&self, elt: Element) -> bool {
+        let can_insert = self.elements_by_id.with(
+            |elts_by_id| !elts_by_id.contains_key(&elt.id)
+        );
+        if can_insert {
+            self.insert_element_unchecked(elt);
+        }
+        can_insert
+    }
+    
     pub fn insert_new_element(&self) {
         // find the next unused identifier in the default sequence
         let mut id_num = 1;
@@ -51,15 +70,15 @@ impl Assembly {
         }
         
         // create and insert a new element
-        let elt = Element {
-            id: id.clone(),
-            label: format!("Sphere {}", id_num),
-            color: [0.75_f32, 0.75_f32, 0.75_f32],
-            rep: DVector::<f64>::from_column_slice(&[0.0, 0.0, 0.0, 0.5, -0.5]),
-            constraints: BTreeSet::default()
-        };
-        let key = self.elements.update(|elts| elts.insert(elt));
-        self.elements_by_id.update(|elts_by_id| elts_by_id.insert(id, key));
+        self.insert_element_unchecked(
+            Element {
+                id: id,
+                label: format!("Sphere {}", id_num),
+                color: [0.75_f32, 0.75_f32, 0.75_f32],
+                rep: DVector::<f64>::from_column_slice(&[0.0, 0.0, 0.0, 0.5, -0.5]),
+                constraints: BTreeSet::default()
+            }
+        );
     }
     
     pub fn insert_constraint(&self, constraint: Constraint) {