As ichabood has stated repeatdely xx is not defined and we/you get name error for xx. Running your latest code:
Error:Traceback (most recent call last):
File "somefile.py", line 15, in <module>
print(fastln(4,4))
File "somefile.py", line 9, in fastln
a0=xx
NameError: name 'xx' is not defined
>>>
If you can't explain it to a six year old, you don't understand it yourself, Albert Einstein
How to Ask Questions The Smart Way: link and another link
Create MCV example
Debug small programs
How to Ask Questions The Smart Way: link and another link
Create MCV example
Debug small programs