Why the dependencies in actionlib tutorial include "roslib"?

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In the introductary tutorial of actionlib package, the sand-box package is created with the dependency roslib. I check up the introduction of roslib package and realize that "roslib contains the definition of the ros::Time and ros::Duration objects used in roscpp and other ROS C++ libraries".

Then I opened a package, wrote a program, utilizing of the ros::Time and ros::Duration without the dependency of roslib. However it worked successfully.

Thus I'm wondering why the dependencies in actionlib tutorial include roslib.

Thanks for answering.