read stream in sbcl

(with-open-stream (files
     (sb-ext:process-output
      (sb-ext:run-program "/usr/bin/cat" '("/path/to/file")
       :output :stream)))
  (loop :for line = (read-line files nil nil)
 :while (and
  (my-mega-condition-p line))
  line) :do (princ line) (terpri)))