I followed cram_language

3.1 is a chapter about fluent, when I type the following

  CPL> (let ((fl (make-fluent :name :test-fluent :value nil)))
    (spawn-perception fl)
    (wait-for fl)
    (format t "Received value: ~a~%" (value fl)))

The error message as following will occur

  The function CRAM-LANGUAGE::SPAWN-PERCEPTION is undefined.
     [Condition of type UNDEFINED-FUNCTION]

But I couldn't find any information about "spawn-perception".

What is the definition of this function? Where can I find the source code of this function?

Thank you.