As any songwriter will tell you, the best lines come to you in the middle of the night.
I can just immagine the author waking up about 2AM and saying "Yes I've got it! And Her Slippers Smelled of Kippers"
When he woke up in the morning and went off to the writting pad and wracked his brain. " yes, Herring Boxes Without topses - hmmm doesn't quite sound the same as the dream!"