public void check() throws InvalidGameStateException {
		if (displayName == null) throw new InvalidGameStateException("category has no displayable name");
		if (fields == null) throw new InvalidGameStateException("category '"+displayName+"' has fields null");
		if (fields.isEmpty()) throw new InvalidGameStateException("category '"+displayName+"' has an empty list of fields");
		
		for (Field field : fields) {
			field.check();
		}
	}