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English version:
Functions are used to perform complex rolls, like exploding die, dice pool, roll & keep and so on.
Functions are composed by their name followed by a set of parameters enclosed in parentheses, and each parameter is separated by a comma. A typical example is "rak(1d6,4,3)" and its meaning is as follows: use the function with name rak with three parameters. First parameter is "1d6", second parameter is "4" and third parameter is "3".
Each parameter of a function can be any formula, even another function, like "max(rak(1d6,4,3),5+2)" which give the highest value between the result of "rak(1d6,4,3)" and "5+2".
To get the complete list of available functions, check the Function Reference page.