How to pass function argument by reference in Circom?

How to pass function argument by reference in the circom circuit language?

I'm trying to do the following:

pragma circom 2.0.0;


function increment(foo) {
    foo++;
}

template MyTemplate() {
    signal input a;
    signal output b;

    var foo;
    foo = 0;

    increment(foo);
    log(foo);


    // ...
}

component main = MyTemplate();

I expect log(pos) to output 1, but I'm getting 0. Is there a certain way I need to pass pos into increment so that it can modify the variable by reference?

I decided to use the C preprocessor to generate circom code, so now I have:

main.circom: 
    cpp -P maintpl.circom > main.circom

in my Makefile

and

#define increment(foo) foo++

in my circom code.