Formal series by formula

Earlier FPS design using generator didn't work.
Here is other using formula based on #7618
some tests from Koepf:
```
In [16]: f = findgen(sin(x)**2+cos(x)**2, x)

In [17]: f(k)
Out[17]: [(0, 0)]

In [18]: f = findgen(cos(asin(x)) - sqrt(1-x**2), x)

In [19]: f(k)
Out[19]: [(0, 0)]
```

Hmm. Something wrong here.