+
+(defun get-properties (plist indicator-list)
+ (do* ((plist plist (cddr plist))
+ (cdr (cdr plist) (cdr plist))
+ (car (car plist) (car plist)))
+ ((null plist) (values nil nil nil))
+ (when (null cdr)
+ (error "malformed property list ~S" plist))
+ (let ((found (member car indicator-list :test #'eq)))
+ (when found
+ (return (values car (cadr plist) plist))))))
+
+(defun getf (plist indicator &optional default)
+ (do* ((plist plist (cddr plist))
+ (cdr (cdr plist) (cdr plist))
+ (car (car plist) (car plist)))
+ ((null plist) default)
+ (when (null cdr)
+ (error "malformed property list ~S" plist))
+ (when (eq indicator car)
+ (return (cadr plist)))))
+
+(defun %putf (plist indicator new-value)
+ (do* ((tail plist (cddr tail))
+ (cdr (cdr tail) (cdr tail))
+ (car (car tail) (car tail)))
+ ((null tail) (list* indicator new-value plist))
+ (when (null cdr)
+ (error "malformed property list ~S" tail))
+ (when (eq indicator car)
+ ;; TODO: should be cadr, needs a defsetf for that
+ (setf (car (cdr tail)) new-value)
+ (return tail))))
+
+(define-setf-expander getf (plist indicator &optional default)
+ (multiple-value-bind (dummies vals newval setter getter)
+ (get-setf-expansion plist)
+ (let ((store (gensym))
+ (indicator-sym (gensym))
+ (default-sym (and default (gensym))))
+ (values `(,indicator-sym ,@(and default `(,default-sym)) ,@dummies)
+ `(,indicator ,@(and default `(,default)) ,@vals)
+ `(,store)
+ `(let ((,(car newval) (%putf ,getter ,indicator-sym ,store))
+ ,@(cdr newval))
+ ,setter
+ ,store)
+ `(getf ,getter ,indicator-sym ,@(and default `(,default-sym)))))))