I've got one.
It's a short story from the 70s or early 80s. Someone is demonstrating has a machine he's invented for writing stories. It's a bit like an organ, with a keyboard and pedals that you can use to select plot devices and add effects such as mystery, romance, or suspense.
The inventor explains it enthusiastically. He seems a bit of an anorak. As the story progresses, you realise that it has been written by the machine.
For example it says something like:- 'you can keep the reader interested by putting in an unusual word at the end of a paragraph' he said, epexegetically.
Any ideas?
Fingers crossed