hark-back vs relapse

hark-back

noun
  • An act of alluding, returning, or reverting (to a subject previously mentioned, etc.); also, an act of evoking, or longing or pining for (a past era or event). 

  • An act of hounds retracing a course in order to pick up the lost scent of prey. 

relapse

noun
  • The act or situation of relapsing. 

  • An occasion when a person becomes ill again after a period of improvement 

verb
  • To slip or slide back physically; to turn back. 

  • To fall back again; to slide or turn back into a former state or practice. 

  • To return to a vice, especially self-harm or alcoholism, failing to maintain abstinence. 

  • To recur; to worsen, be aggravated (after a period of improvement). 

How often have the words hark-back and relapse occurred in a corpus of books? (source: Google Ngram Viewer )