Unexpected ERROR: UndefVarError: a not defined

I think the first step here would be to paste your code in a way we can run in our machines. I realize this could be a bit more borhersome, but it will allow us to help you more quickly (and, we really want to since we’re a nice community! :slight_smile: )

For example

your code should be inside this 

Some other useful guidelines on how to post code, use discourse most efficiently etc. are also given here:

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