Don't test for Number (abstract class)

Before:
```
$ ./expand2
Expanding: ((y + x + z + w)^15 + w)*(y + x + z + w)^15
1565ms
number of terms: 6272
```
After:
```
$ ./expand2
Expanding: ((y + x + z + w)^15 + w)*(y + x + z + w)^15
1519ms
number of terms: 6272
```