public Set<String> getNullEntries() {
        final Set<String> result = new HashSet<>();
        for (Map.Entry<String, VariableState> entry : variableStateMap.entrySet()) {
            if (entry.getValue().formulaString == null) {
                result.add(entry.getKey());
            }
        }
        return result;
    }
Returns all variables in this map which have a null state (e.g. are not explicitely created but only as dependencies of other vars
main/src/main/java/cgeo/geocaching/utils/formulas/VariableMap.java