Roughly speaking, it is as if the defstruct form

(defstruct op “An operation”

 (action nil) (preconds nil) (add-list nil) (del-list nil))

expanded into the following definitions:

(defun make-op (&key action precondsadd-list del-list)

 (vector ‘op action preconds add-list del-list))

(defun op-action (op) (elt op 1))

(defun op-preconds (op) (elt op 2))

(defun op-add-list (op) (elt op 3))

(defun op-del-list (op) (elt op 4))

(defun copy-op (op) (copy-seq op))

(defun op-p (op)

 (and (vectorp op) (eq (elt op 0) ‘op)))

(setf (documentation ‘op ‘structure) “An operation”)