code: public ListBuffer<A> append(A x) {
          Assert.checkNonNull(x);
          if (shared) copy();
          List<A> newLast = List.of(x);
          if (last != null) {
              last.tail = newLast;
              last = newLast;
          } else {
              elems = last = newLast;
          }
          count++;
          return this;
      }
docs: Append an element to buffer.