using drupal_process_states() I'm trying to add dependencies to a set of fields, such that only one of the set can ever contain a value at any given time.

I see that drupal_process_states() does pretty much what I want, but I'd have to define it separately for each element. It looks like I'd be using it with hook_field_widget_WIDGET_TYPE_form_alter(), since that takes $elements as an argument?

Is there anything that might simplify this? Specifically, is there a function that can apply rules per fieldset rather than per widget?
I think I was confused about what drupal_process_states() does. It does not have to be declared like a hook or even invoked in my custom module. It is simply the machinery behind #states in the FormAPI. Simply using a hook_form_alter() and adding a #states array to the relevant fields is enough.