harbinger vs prewarn

harbinger

verb
  • To announce or precede; to be a harbinger of. 

noun
  • A person or thing that foreshadows or foretells the coming of someone or something. 

prewarn

verb
  • To warn beforehand; to forewarn. 

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