I want to make the tensor product of polynomial algebras. Evidently, the following is not the right way to do this:

```
sage: P.<x> = PolynomialRing(QQ)
sage: tensor([P, P])
---------------------------------------------------------------------------
AssertionError Traceback (most recent call last)
<ipython-input-2-17decd317813> in <module>()
----> 1 tensor([P, P])
/usr/lib/python2.7/site-packages/sage/categories/covariant_functorial_construction.pyc in __call__(self, args, **kwargs)
219 """
220 args = tuple(args) # a bit brute force; let's see if this becomes a bottleneck later
--> 221 assert(all( hasattr(arg, self._functor_name) for arg in args))
222 assert(len(args) > 0)
223 return getattr(args[0], self._functor_name)(*args[1:], **kwargs)
AssertionError:
```

This does work for `CombinatorialFreeModule`

s but I can't figure out how to turn one of those into a polynomial algebra.