code: @Override
  	public FieldList addField(Integer fieldID) {
  		if (fieldID == null) {
  			throw new IllegalArgumentException("Field ID must not be null.");
  		}
  		
  		if (size() == 0) {
  			return new FieldList(fieldID);
  		} else {
  			ArrayList<Integer> list = new ArrayList<Integer>(size() + 1);
  			list.addAll(this.collection);
  			list.add(fieldID);
  			return new FieldList(Collections.unmodifiableList(list));
  		}
  	}
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