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make-broadcast-stream  &rest streams ⇒  broadcast-stream
stream—an output stream.
broadcast-stream—a broadcast stream.
Returns a broadcast stream.
 (setq a-stream (make-string-output-stream)
        b-stream (make-string-output-stream)) ⇒  #<String Output Stream>
 (format (make-broadcast-stream a-stream b-stream)
          "this will go to both streams") ⇒  NIL
 (get-output-stream-string a-stream) ⇒  "this will go to both streams"
 (get-output-stream-string b-stream) ⇒  "this will go to both streams"
Should signal an error of type type-error if any stream is not an output stream.