flagpole vs keep at

flagpole

verb
  • Exit a country momentarily and reenter. Usually this is done to satisfy immigration requirements. 

noun
  • A tall pole up which one or more flags may be raised and flown. 

keep at

verb
  • To persist in. 

  • To constrain (a person) to continue doing (a task). 

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