public void writeElement(String key, Object value)
throws IOException, SaveGameException {
assert isResponsible(value);
Persistent persistent = (Persistent)value;
write("<" + Defs.getPersistentCode() + Defs.getSeparator() + key);
changeIndentLevel(1);
DataToSave subData = persistent.getPersistentData();
if (subData != null) {
writeMap(subData);
} else {
throw new SaveGameException("Error: Persistent " + persistent + " returns "
+ "null instead of a data-map!");