To define a function, we have to use the fun keyword; this also can be declared as a first class citizen. This means that a function can only be defined in a file. We will touch on functions in greater detail in the Functions section, but for now, let's look at a simple example that changes the value of the mutable variable:

fun changeMutable() {
    mutable = 4
}

In the previous snippet, we can see that the changeMutable function can be declared as a first class citizen in the same file as the mutable variable, or in any other place.