py2many.exceptions.AstNotImplementedError: sequence item 0: expected str instance, NoneType found
Hi,

I'm invoking:

```
09:44:42 1d folkert@belle:~/git/bf-compiler master(2)* ± py2many --cpp=1 mandelbrot.py --outdir t/
Writing to: t
mandelbrot.py ... t/mandelbrot.cpp
mandelbrot.py:14:0: py2many.exceptions.AstNotImplementedError: sequence item 0: expected str instance, NoneType found
```

[mandelbrot-python.zip](https://github.com/py2many/py2many/files/9225635/mandelbrot-python.zip)
