Explicit Options

Created by: patinthehat

I'd like to see a way to enable explicit options, meaning either option "x" can be passed as an option, but option "y" would not be allowed or vice-versa (with one possibly being the default).
A real-world example might be: script.php --read myfile.txt or script.php --write myfile.txt "test data" The idea is that both options cannot coexist ahd either an Exception would be thrown or it would default to one. It could even be expanded to multiple arguments or operands.

Any thoughts?