code: public void setValue(Token[] tokens) {
          if (type == null) {
              type = ItemType.SINGLE;
          }
          if (type != ItemType.SINGLE) {
              throw new IllegalArgumentException("The type of this item must be 'single'");
          }
          this.tokens = tokens;
      }
docs: Sets the specified value to this Single type item.
  
  @param tokens an array of tokens