(uiop:with-upgradability ()
  (defclass fw-define-op (asdf:define-op)
    ((%systems-before :initarg :systems-before :reader systems-before)
     (%new-systems :initarg :new-systems :accessor new-systems)))
  (defmethod asdf:operate :before ((o fw-define-op) (c asdf:system) &key)
    (setf (slot-value o '%systems-before) (asdf:registered-systems)))
  (defmethod asdf:operate :after ((o fw-define-op) (c asdf:system) &key)
    (setf (new-systems o) (set-difference (asdf:registered-systems)
                                          (slot-value o '%systems-before)
                                          :test 'equal)))
  (defun load-asd (pathname &key name)
    "Load system definitions from PATHNAME.

(defclass fw-define-op (asdf:define-op)
  ((%systems-before :reader systems-before :initform (asdf:registered-systems))
   (%new-systems :initarg :new-systems :accessor new-systems)))
(defmethod asdf:operate :after ((o fw-define-op) (c asdf:system) &key)
  (setf (new-systems o) (set-difference (asdf:registered-systems)
                                        (slot-value o '%systems-before)
                                        :test 'equal)))
(defun load-asd (pathname &key name)
  "Load system definitions from PATHNAME.

    (asdf/session:with-asdf-session ()
      ;; TODO: use OPERATE, so we consult the cache and only load once per session.
      (flet ((do-it (o c) (asdf:operate o c)))
        (let ((primary-name (asdf:primary-system-name (or name (pathname-name pathname))))
              (operation (asdf:make-operation 'fw-define-op)))
          (uiop:if-let (system (asdf:registered-system primary-name))
            (progn
              ;; We already determine this to be obsolete ---
              ;; or should we move some tests from find-system to check for up-to-date-ness here?
              (setf (asdf/action:component-operation-time operation system) t
                    (asdf/system:definition-dependency-list system) nil
                    (asdf/system:definition-dependency-set system)
                    (uiop:list-to-hash-set nil))
              (do-it operation system))
            (let ((system (make-instance 'asdf/system:undefined-system
                                         :name primary-name :source-file pathname)))
              (asdf/system-registry:register-system system)
              (unwind-protect (do-it operation system)
                (when (typep system 'asdf/system:undefined-system)
                  (asdf:clear-system system))))))))))

  (asdf/session:with-asdf-session ()
    ;; TODO: use OPERATE, so we consult the cache and only load once per session.
    (flet ((do-it (o c) (asdf:operate o c)))
      (let ((primary-name (asdf:primary-system-name (or name (pathname-name pathname))))
            (operation (asdf:make-operation 'fw-define-op)))
        (uiop:if-let (system (asdf:registered-system primary-name))
          (progn
            ;; We already determine this to be obsolete ---
            ;; or should we move some tests from find-system to check for up-to-date-ness here?
            (setf (asdf/action:component-operation-time operation system) t
                  (asdf/system:definition-dependency-list system) nil
                  (asdf/system:definition-dependency-set system)
                  (uiop:list-to-hash-set nil))
            (do-it operation system))
          (let ((system (make-instance 'asdf/system:undefined-system
                                       :name primary-name :source-file pathname)))
            (asdf/system-registry:register-system system)
            (unwind-protect (do-it operation system)
              (when (typep system 'asdf/system:undefined-system)
                (asdf:clear-system system)))))))))