我写的第一份跟自定义流有关的代码

CL-USER> (setf path (make-pathname :name "BECKY"))
#P"BECKY"
CL-USER> (with-open-file (bstr path :direction :output
                   :if-exists :supersede)
       (format bstr "hello world!~%"))
;Compiler warnings :
;   In an anonymous lambda form: Undeclared free variable PATH
NIL