Constraints on classes with many attributes that can create constraints **between** attributes.

[karishmadaga]

Example:

@constraint.exactly_one(e, groupby="a")
class A(object):
    def __init__(self, a, b):
        self.a = a
        self.b = b

Such that for each class object, we create a constraint exactly_one between all instances that share the same value for self.a.

TODO: Identify the following,
What should the user be able to do with this feature?

What would a user flow look like for this?

What would the expected output be?

[haz]

This is closed with #83 , no?