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make-echo-stream  input-stream output-stream ⇒  echo-stream
input-stream—an input stream.
output-stream—an output stream.
echo-stream—an echo stream.
Creates and returns an echo stream that takes input from input-stream and sends output to output-stream.
 (let ((out (make-string-output-stream)))
    (with-open-stream 
        (s (make-echo-stream
            (make-string-input-stream "this-is-read-and-echoed")
            out))
      (read s)
      (format s " * this-is-direct-output")
      (get-output-stream-string out)))
⇒  "this-is-read-and-echoed * this-is-direct-output"
echo-stream-input-stream , echo-stream-output-stream, make-two-way-stream