(defun system-checksum (system &optional (digest 'ironclad:sha256))
  "Return a checksum of files in a system."
  (let (streams)
    (flet ((open-thing (c)
	    (let ((s (open (if (pathnamep c) c
			       (asdf/component:component-pathname c))
		     :direction :input
		     :element-type '(unsigned-byte 8))))
	      (push s streams)
	      s)))
      (unwind-protect
	  (ironclad:digest-stream
	   digest
	   (apply #'make-concatenated-stream
		  (mapcar #'open-thing
		   (append
		    (asdf/component:component-children
		     (asdf/system:find-system system))
		    (list
		     (asdf:system-source-file (asdf:find-system system)))))))
	(map nil (lambda (s) (close s)) streams)))))