Modifying constraints

Hiya - this really is an awesome library - thx for this. 

Quick one - I see in the API docs that constraints can be added, however it looks like modifying / removing isn't a use case that's covered. Is this correct ?

If this is the case I'll try to wade through the code to see if I can figure out how to add this. I just thought I'd ask before hacking stuff since I'm lazy that way ;-P