> 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?

`engine-null` solves both the prefix issue and mirrors `eof-object`, so
I'd go with that.

> It can go either way: R6RS consistently uses a procedure, but since it is
> an error (i.e. undefined behavior) to mutate an imported variable, it's
> okay to just bind it with `define` and not bother with the procedure.

True, but if it's less confusing to consistently use procedures
throughout the SRFI, I don't see why not.

Vasilij