clef vs ghostwrite

clef

noun
  • A symbol found on a musical staff that indicates the pitches represented by the lines and the spaces on the staff 

ghostwrite

verb
  • To write under the name of another (especially literary works). 

  • To author a literary work or speech in the place of another. 

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