go ahead vs nullify

go ahead

verb
  • To proceed with; to begin. 

  • Used to grant permission for or to give endorsement of a suggestion or proposal. 

  • To send communication over the radio. 

nullify

verb
  • To prevent from happening. 

  • To make of no use or value; to cancel out. 

  • To make legally invalid. 

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