Implement projection onto the zero subspace

app-proto/src/engine.rs

@@ -141,14 +141,16 @@ impl ConfigSubspace {
     
     // find the projection onto this subspace of the motion where the element
     // with the given column index has velocity `v`
-    /* TO DO */
-    // for the zero subspace, this method's behavior doesn't match its name: it
-    // panics rather than returning zero
     pub fn proj(&self, v: &DVectorView<f64>, column_index: usize) -> DMatrix<f64> {
+        if self.dim() == 0 {
+            const ELEMENT_DIM: usize = 5;
+            DMatrix::zeros(ELEMENT_DIM, self.assembly_dim)
+        } else {
             self.basis.iter().map(
                 |b| b.column(column_index).dot(&v) * b
             ).sum()
         }
+    }
 }
 
 // --- descent history ---