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b in an Interactive Expression

The next argument in the interactive expression starts with a lower case b. (This is different from the code for append-to-buffer, which uses an upper-case B. See The Definition of append-to-buffer.) The lower-case b tells the Lisp interpreter that the argument for insert-buffer should be an existing buffer or else its name. (The upper-case B option provides for the possibility that the buffer does not exist.) Emacs will prompt you for the name of the buffer, offering you a default buffer, with name completion enabled. If the buffer does not exist, you receive a message that says "No match"; your terminal may beep at you as well.