721
I dunno
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5 isn't a valid function name, is obviously the right answer.
How can you be sure it's not defined when we only see one line?
They didn't say it's not defined, they said it's not a valid name. Most languages don't allow function names to start with a number, so 5 literally cannot be a function if that's the case.
But that's assuming this isn't some really obscure language.
It could be a Church Numeral
Depends on the language.
I'm pretty sure that's a module operator...