@NonNull
    public String getAsString(final int idx, final String defaultValue) {
        return isValidIdx(idx) ? list.get(idx).getAsString() : defaultValue;
    }
Encapsulates a list of {@link cgeo.geocaching.utils.formulas.Value}'s for handling as parameter lists in {@link Formula}
main/src/main/java/cgeo/geocaching/utils/formulas/ValueList.java