preliminary vs prolog

preliminary

noun
  • A preparation for a main matter; an introduction. 

  • Any of a series of sports events that determine the finalists 

  • A relatively minor contest that precedes a major one, especially in boxing 

adj
  • In preparation for the main matter; initial, introductory, preparatory. 

prolog

noun
  • A speech or section used as an introduction, especially to a play or novel. 

  • A component of a computer program that prepares the computer to execute a routine. 

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