code: byte[] popOne() throws GeneratorException {
          try {
              return idStack.pop();
          } catch (NoSuchElementException e) {
              // Cached stack is empty, load up a fresh stack.
              idStack.addAll(generator.batch(batchSize));
              return popOne();
          }
      }
docs: Grab a single ID from the stack. If the stack is empty, load up a new batch from the wrapped generator.
  
  @return A single ID.