immure vs pen in

immure

verb
  • To cloister, confine, imprison or hole up: to lock someone up or seclude oneself behind walls. 

  • To put or bury within a wall. 

  • To wall in. 

  • To trap or capture (an impurity); chiefly in the participial adjective immured and gerund or gerundial noun immuring. 

pen in

verb
  • To cause to be stuck in a situation; to be forced into a situation by circumstances. 

  • To enclose (animals) in a pen. 

  • To schedule; fix a date for. 

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