@Nullable
    public Character getLowestMissingChar() {
        if (!asSet().contains("A")) {
            return 'A';
        }
        int lowestContChar = 'A';
        while (lowestContChar < 'Z' && asSet().contains("" + (char) (lowestContChar + 1))) {
            lowestContChar++;
        }
        return lowestContChar == 'Z' ? null : (char) (lowestContChar + 1);
    }
Tidies up this variable list. This means doing the following things:
* remove empty entries without dependency of any other var (or from varsToKeep list)
* add empty entries for all missing vars
* sorts vars alphabetically
main/src/main/java/cgeo/geocaching/utils/formulas/VariableList.java