Problem Statement

Obtain a valid   Y   or   N   response from the keyboard. The keyboard should be flushed, so that any outstanding key-presses are removed, preventing any existing   Y   or   N   key-press from being evaluated. The response should be obtained as soon as   Y   or   N   are pressed, and there should be no need to press an   enter   key.

Let's start with the solution:

(de yesno ()
   (loop
      (NIL (uppc (key)))
      (T (= "Y" @) T)
      (T (= "N" @)) ) )