Polish Rational

Master (a28e678):
```
$ ./expand2
Expanding: ((y + x + z + w)^15 + w)*(y + x + z + w)^15
1539ms
number of terms: 6272
$ ./expand2
Expanding: ((y + x + z + w)^15 + w)*(y + x + z + w)^15
1534ms
number of terms: 6272
```

As of 17014346a0310e5e0345d401c3f336dc4fb96048:
```
$ ./expand2
Expanding: ((y + x + z + w)^15 + w)*(y + x + z + w)^15
1553ms
number of terms: 6272
```
So a slowdown of of 0.9% or 1.2%, depending on which of the times from master we use.