Explain why the empty-assembly case is special

app-proto/src/engine.rs

@@ -383,7 +383,9 @@ pub fn realize_gram(
     // start the descent history
     let mut history = DescentHistory::new();
     
-    // handle the empty-assembly case
+    // handle the case where the assembly is empty. our general realization
+    // routine can't handle this case because it builds the Hessian using
+    // `DMatrix::from_columns`, which panics when the list of columns is empty
     let assembly_dim = guess.ncols();
     if assembly_dim == 0 {
         let result = Ok(