enroll vs expel

enroll

verb
  • To enlist oneself (in something) or become a member (of something) 

  • To enter (a name, etc.) in a register, roll or list 

  • To enlist (someone) or make (someone) a member of 

expel

verb
  • To eject or erupt. 

  • To remove from membership. 

  • To deport. 

How often have the words enroll and expel occurred in a corpus of books? (source: Google Ngram Viewer )