### Exponent overflow in PolynomialRing(): need a work around

PolynomialRing() gives an OverflowError for exponents larger than 32768.
For example

```
sage: R = GF(2**28, 'a')
sage: a = R.gen()
sage: x = PolynomialRing(R, 'x', 4).gens()
sage: f = x[0]**32768
sage: f = x[0]**32769
...
OverflowError: Exponent overflow (32769).
```

I need to make a function containing `x[0]**(2**28 - 2)`

.
How can I get Sage to do that?