What has the author Cay Hehner written?

It seems you might be mistaken about the author's name. There is no widely known author named "Cay Hehner."

Perhaps you're thinking of:

* Eric Hehner: A computer scientist known for his work in formal methods, programming languages, and mathematics. He has written several books, including "A Practical Theory of Programming" and "The Logic of Programming".

* Cay Horstmann: A computer science author well-known for his Java programming books, including "Core Java" and "Java SE 11 for Programmers".

Could you please double-check the name of the author you are looking for? Providing more context, like the genre or the topics they write about, might also be helpful.