code: public TimeCounter stop() {
          switch (state) {
              case UNSTARTED:
                  throw new IllegalStateException("Not started. ");
              case STOPPED:
                  throw new IllegalStateException("Already stopped. ");
              case RUNNING:
                  watch.suspend();
          }
          state = State.STOPPED;
          return this;
      }
docs: Stops or suspends the time count.
  
  @return This {@link TimeCounter}.