Hello again,

> I think when an immutable object is exported from a library, it must
> be done as a procedure like eof-object.
> So in this case, the procedure `engine-null` seems to do the job?

I've looked at this now and discovered that it must be a procedure as
the null value is unique to the engine and should ideally have a
(engine-null engine) function signature. However that would mean
extending the engine record accordingly and adjusting the documentation
in a few more places. Is that acceptable for you?