This looks good. But we should add a comment in line 85 as well,
see
```
In [3]: p = Poly(x**3 + k*x*4 + 1, x)

In [4]: RootOf(p, 0)
---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
<ipython-input-4-22d3d05a9408> in <module>()
----> 1 RootOf(p, 0)

/home/sophie/sympy/sympy/polys/rootoftools.pyc in __new__(cls, f, x, index, radicals, expand)
     83 
     84         if not dom.is_ZZ:
---> 85             raise NotImplementedError("RootOf is not supported over %s" % dom)
     86 
     87         root = cls._indexed_root(poly, index)

NotImplementedError: RootOf is not supported over ZZ[k]
```