'What to put in blanks to complete python program (recursion)

I've been at it for hours now yet I still don't know what to put in these blanks. I have an idea on how it will work but I don't know how to apply it. Help!

Basically, I need to find out what are the blanks in order for the program to run. It's a recursion problem where the program would just basically get the sum of all the elements in the list.

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