for (Object key : newProps.keySet()) {
if (!String.class.isInstance(key)
&& key.toString().startsWith(prefix))
throw new UserException(_loc.get("slice-add-wrong-key", key));
}
Slice slice = getSlice(name);
if (slice != null)
throw new UserException(_loc.get("slice-exists", name));
Map<String,String> original = super.toProperties(true);
original.putAll(newProps);
slice = newSlice(name, original);