quad_potential will not work here. I think I would just remove it and have normal_proposal be the default here. quadpotential was already kind of awkward here. If you really want it, make a function like 

```
def quadpotential_proposal(hess, is_cov ... ): 
    p = quad_potential(...)
    return lambda : p.random()  //except don't use a lambda
```

But I think it's totally fine to remove it.