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:

on KeyDown k
    if toUpper(k) is among the items of "Y,N" then 
        answer "Thanks for your response"
    else
        answer "You need to enter Y or N"
    end if
    put empty into me
end KeyDown