List myAverage := method(
    total := 0
    if (size == 0) then (
        nil
    ) else (
        foreach(x,
            if (x type == "Number") then (
                total = total + x
            ) else (
                Exception raise("Cannot get the average of non-numbers!")
            )
            total / size
        )
    )
)

list(1, 2, 3, 4) myAverage println      // outputs 2.5
list() myAverage println            // outputs nil
list(2, 3, 4) myAverage println         // outputs 3
list(2, "three", 4) myAverage println       // throws an exception