Dave suggests:
> rewriting. The problem comes with preserving old values. I haven't
> worked out how to do that without a change to the interpreter.

Or maybe he should suggest :). What do you guys think, would it be step to
right direction to add DBC to the language (or to the interpreter).

(Here I have to confess I haven't even thought about preserving the old
values for post-check...)

	- Aleksi