bonds vs helotism

bonds

noun
  • imprisonment, captivity 

  • the condition of goods in a bonded warehouse until duty is paid 

helotism

noun
  • The permanent oppression of a nominally free group of people 

  • A form of mutualism in which one species is forced to perform tasks for another, for their mutual benefit 

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