> Why don't you just define these certain variables such that you get a > run-time exception if you attempt to reference them? Your exception > handler can then recognize this exception and handle it appropriately, > e.g., by aborting the computation. This is how I usually define these > not-yet-defined-really variables.
I will try to take up this suggestion and see if it can work. One problem I immediately run into is that some of the exports that may or may not be present are macros (and others are values). Presumably I would have to define them all as macros that expand either to raising the exception or to referencing the imported value... but how to reference an imported macro?
