@Redirect(method = "incrementX", at = @At(value = "INVOKE", target = "Lpath/to/Crementer;crement(I)I"))
private int actuallyDecrement(
    Crementer in, // the target's class instance
    int x,        // the target's arguments
    int by        // incrementX's arguments
) {
    Crementer de = new Crementer(-by);
    return de.crement(x); // >:)
}