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file-length  stream ⇒  length
stream—a stream associated with a file.
length—a non-negative integer or nil.
file-length returns the length of stream, or nil if the length cannot be determined.
For a binary file, the length is measured in units of the element type of the stream.
 (with-open-file (s "decimal-digits.text" 
                    :direction :output :if-exists :error)
   (princ "0123456789" s)
   (truename s))
⇒  #P"A:>Joe>decimal-digits.text.1"
 (with-open-file (s "decimal-digits.text")
   (file-length s))
⇒  10
Should signal an error of type type-error if stream is not a stream associated with a file.