A commentator on my entry on why your main program should be import able mentioned that a third reason is to beable to unit test your program. I agree with the sentiment in general,but in specific I've never been able to get unit testing of my mainprogram code to work very well.

While I can unit test most of the low-level code involved in a program,I've found that much of the high level code, the stuff that goes in themain program, has two