It depends completely on whether you are looking for "valid" in a structural sense or a semantic sense. If a structural sense, lutusp's second approach looks v

It depends completely on whether you are looking for "valid" in a structural sense or a semantic sense. If a structural sense, lutusp's second approach looks very do-able. There are a lot of cases to consider and it would most likely mean pulling out an old grammar book that actually teaches these things but it could be approached systematically. It also would mean making some assumptions about verb forms, ie I would only want to have to choose the verb at random and not the form. If "valid" in a semantic sense, I wouldn't have a real clue where to start. The structural requirements give you an upper bound and a quick initial test. Perhaps you could start with a small set of words and observed how they scale upwards?