Thanks for a fantastic library!

One thing that surprised me at first was that I could not write an effective __gt__()-function for my attr-class. I found the cmp-init-parameter later (in the code), but I still wonder if it wouldn't be a good idea if I could have the option of keeping the tuple-based __eq__ and __ne__-implementations, but override size-comparisons (maybe based on a single __lt__-function?).