[magicmouse]

Excel achieves its robustness (and it hardly ever crashes) by protecting the user against arithmetic errors, and circular reasoning. Excel doesn't offer infinite precision arithmetic, that is rarely needed in practice.

Null pointers are something that plagues Java, which are primarily caused by having some computation out of sequence. By using the State-Action-Model pattern, most of Beads code will be devoted to rendering the model on the screen, and that one-way transfer of information can be made very robust, provided you have a closed arithmetic, which it does.