The class fundamental-character-input-stream
provides default methods for generic functions used for character input, and should therefore be included by stream classes concerned with character input. The user can provide methods for these generic functions specialized on the user-defined class. Methods for other generic functions must be provided by the user.
There is an example in Defining a new stream class.
fundamental-character-stream
fundamental-input-stream
stream-clear-input
stream-listen
stream-peek-char
stream-read-char
stream-read-char-no-hang
stream-read-line
stream-read-sequence
stream-unread-char
LispWorks User Guide and Reference Manual - 20 Sep 2017