Issues implementing a "Not" comparison

Basically, I tried implementing a generic `Not` comparison like `assert.Assert(t, Not(cmp.Equal(foo, bar))`.

The interface here seems a bit limiting as I do not have access to variables, formatting or anything from my `Not` comparison.
It seems right now I would have to implement a `NotEqual` comparison in order to gain access variables and the like.

I'm wondering if some additional interface could help here? Maybe something that a comparison could optionally implement to make it easier to wrap?