[Vulume]

In Darker than Black there are Contractors. They have a "contract" which means they can cast one particular spell, after which they have to complete their "contract payment" before they can cast again. That's often a simple but annoying task like smoking a cigarette or writing a poem. The spell and payment are different for each one. Nobody got to choose.

http://darkerthanblack.wikia.com/wiki/Obeisance

[comrh]

That is such an interesting idea, especially because so many of those "obeisances" mimic OCD symptoms.