Cross-validation of dependent traits

Suppose I have an object with traits 'a', 'b', and 'c', with the constraints that a>b>c and default values a=1, b=10, c=100. I'd like to write a validator that checks this constraint, such that I can assign all three values at once to get a new consistent state (either on creation, like MyObject(a=5, b=6, c=7). How would I write such a validator?

If it's not possible, would it be possible to include in the proposal argument to a validator the new values of any other traits in the same assignment, so I can compare against what the object state will be instead of what it currently is?