asserts and comment

Resample/Processed/ReSample.cpp

@@ -128,12 +128,14 @@ SliceStack slicify(const MultiLayer& sample, const SimulationOptions& options)
             if (roughness && roughness->sigma() <= 0)
                 roughness = nullptr;
 
-            if (i == nLayers - 1)
+            if (i == 0) {
                 ASSERT(tl == 0);
-            if (i == 0)
                 result.addTopSlice(tl, *material);
-            else
+            } else {
+                if (i == nLayers - 1)
+                    ASSERT(tl == 0);
                 result.addSlice(tl, *material, roughness);
+            }
         }
         return result;
     }
@@ -335,7 +337,7 @@ ReSample ReSample::make(const MultiLayer& sample, const SimulationOptions& optio
 
     const bool polarized = forcePolarized || sample.isMagnetic() || sample.externalField() != R3();
 
-    // stack1: accounting for roughness, but not yet for particles
+    // if requested, slice layers that contain particles
     const SliceStack stack1 = slicify(sample, options).setBField(sample.externalField());
 
     std::vector<ReLayout> layouts = collectLayouts(sample, stack1);