handler vs substituend

handler

noun
  • A subroutine that handles a particular situation such as an event or exception. 

  • One who handles something (especially manually) or someone. 

  • An advisor or manager to a person occupying a position or office to which the speaker believes the holder does not possess the typical qualifications and/or experience. 

  • A controller, trainer, someone who handles a specified thing, animal or person (especially a prizefighter). 

substituend

noun
  • A substituendum; something to be substituted or replaced. 

  • A substitute; something that substitutes for another. 

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