How can use a variable inside a function to point to an argument

labelList

and then use that variable to modify to the original list? Something like this, although this is not compiling

int foo(const int i, labelList& x1, labelList& x2)
{
    labelList x;
    if (i == 0)
        x = & x1;
    else
        x = & x2;
    x[0] = 1;
}