This interface permits the definition of new classes of streams, and programming their behavior by defining methods for the elementary stream operations. It is based on the proposal STREAM-DEFINITION-BY-USER:GENERIC-FUNCTIONS of David N. Gray to X3J13 and is supported by most Common Lisp implementations currently in use.

All symbols defined by this interface, starting with the prefix FUNDAMENTAL- or STREAM-, are exported from the package "GRAY" and EXT:RE-EXPORTed from "EXT".