[boost] [program options] optional/required options

Hi, I don't know if this has been answered yet, but is there any concept of optional/required options in the program option library? Currently, only "allowed options" can be defined, no? I would be great if you could define different sets of required option groups (because it's possible that a+b _or_ a+c are required) and provide some validatation for that. Otherwise, you have to do that by hand again and again. Stefan