Hi, I tried to save a supplied-p parameter from a BOA constructor like this: (defstruct (delayed-iseq (:constructor iseq (start-or-end &optional (end 0 end?) (by 1) strict-direction?))) "Delayed index sequence evaluation." start-or-end end end? by strict-direction?) but SBCL complained that end? was not used. After rereading the CLHS page for defstruct, I am still not 100% sure why this is. My intepretation is that END? is an argument like any other, but probably I am not getting something. Anyhow, instead I used (defstruct delayed-iseq "Delayed index sequence evaluation." start-or-end end end? by strict-direction?) (defun iseq (start-or-end &optional (end 0 end?) (by 1) strict-direction?) (make-delayed-iseq :start-or-end start-or-end :end end :end? end? :by by :strict-direction? strict-direction?)) Is there a way to do it in a single defstruct definition? Best, Tamas PS.: (eq end nil) is different from when it is not supplied, hence the need to save end?.