[Jprogramming] paren question

[John Wilson]

In the following, why does "paren" parse "i." as a monad?

   load 'trace'
   assert 'noun' -: >type 'b'
   assert 'noun' -: >type 'z'
   assert 'verb' -: >type 'sort'
   paren 'z i. sort sort each b'
(z (i. (sort (sort each) b)))

/John Wilson