code: public void push( Token token ) {
          size++;
          if( first == null ) {
              first = token;
              last = token;
              token.previous = null;
              token.next = null;
          } else {
              last.next = token;
              token.previous = last;
              token.next = null;
              last = token;
          }
      }
docs: Adds a new Token to the end of the linked list