Before revision:  A free boundary in a region of negative source -- an ablation-caused margin -- turns out to be a barrier to exact balance for a numerical scheme ( in either a continuous- or discrete-space sense ). We propose computablea posteriori quantities which allow conservation-error accounting in finite volume and  element schemes.
After revision:  A free boundary in a region of negative source -- an ablation-caused margin -- turns out to be a barrier to exact balance for a numerical scheme ( in either a continuous- or discrete-space sense a posteriori quantities which allow conservation-error accounting in finite volume and element schemes.