In reply to:

Actually would USns have any means of distinguishing between "donkey" (correct interpretation) and "arse" (incorrect, but equally apt) in a statement such as the one above? And would this be why you used an "a" rather than "an", thus specifying donkey? From a position of ignorance this does seem an eensy bit plausible...

well, that's the whole joke, isn't it? their interchangeability?
and as far as "a" and "an" go, that's all about the vowel sound that follows, no?


formerly known as etaoin...