It's an operator(like "+", "-", "*", "/"), not a function. Try something like this:
tick_count = 3
if tick_count mod 3 = 0 then
doSomethingHere
end if
Also I think you should be able to use 'NOT val'. The parentheses just indicate order of operation (e.g. "not(b>40)" first compares b to 40 then NOTS the result).
/edit: Also see RenoJim's comment below.
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