Method: (PRINT-OBJECT SOURCE-FILE-PART T)

Source

(defmethod print-object ((part source-file-part) stream)
  (print-unreadable-object (part stream :type t :identity t)
    (format stream "~S" (if (< (length (text part)) 10)
                            (text part)
                            (strcat (subseq (text part) 0 10) "...")))))