[mannykannot]

I vaguely recall Fred Brooks, in The Mythical Man-Month, using a somewhat similar situation, but involving the various US states' income tax rules and their relationship to the federal tax, as an example in order to make some sort of point (that point being 'know your data', IIRC.)

In a situation where there is a base model with specific modifications - which is, I feel, how airspace regulation mostly works - then I suspect that a DRY approach would make it easier to inspect and test, so long as it stays that way.