title: Can I extend actor in use case?
body: <p>Can I extend actor in use case?
  for example I have "normal user" , "student user" and "teacher user"...
  every use case the "normal user" uses it ,by default the "student user" and "teacher user" use it too..
  so can I extend "normal user" for "student user" and "teacher user" ? is that popular ?
  Thanks a lot :)</p>