Formal series by formula

Earlier FPS design using generator didn't work.
Here is other using formula based on #7618
Given that your `gen` is not supposed to be in a form `f(k)*x**k` - are you sure this is still valid?

If it's - your `gen` doesn't have any specific x-dependence, not even something like `f(k)*x**k+f(k+m)*x**(k+m)+...` (where `m` - some Integer).