The type of the object to return by reference.
If non-nil, if the input argument is nil
a null pointer is passed instead of a reference to an object containing nil
.
The FLI type :reference-return
is equivalent to:
(:reference :lisp-to-foreign-p nil
:foreign-to-lisp-p t)
See :reference for the details.
LispWorks Foreign Language Interface User Guide and Reference Manual - 29 Sep 2017