> (make-foreign-error
>   :set 'errno
>   :code 'ETOOMANYREFS
>   :message "Too many references: can't splice")

(foreign-error-ref ferr 'code) => 'ETOOMANYREFS

The usage of keywords vs symbols in the above is equivalent to the
following Common Lisp idioms:

(define-condition foreign-error/errno (error)
   ((set     :initarg :set)
    (code    :initarg :code)
    (message :initarg :message)))

;; make-instance (akin to make-foreign-error) uses keywords:
(make-instance
  'foreign-error/errno
  :set 'errno
  :code 'ETOOMANYREFS
  :message "Too many references: can't splice")

;; slot-value (akin to foreign-error-ref) uses symbols:
(slot-value e 'message) => "Too many references: can't splice"