default values *and* option callbacks

The current implementation of option callback functions leaves the option value, if the option has not been seen in the arguments, as the callback function. Once it has been seen, the callback function is called, and the option value is set to the return value of the called function, fundamentally different from before.

This is unsatisfactory. What I'd like is to specify the callback and and a default value. Maybe the callback should go in place of the type, not the default? I tried that and decided against it, but I don't remember the reason.

Must re-evaluate.