Because Common Lisp performs its input and output on objects called streams , CLIM does, too. In CLIM, streams are specialized sheets that implement the sheet and stream protocols. The basic stream protocols for input and output provide fundamental functionality such as reading and writing characters and flushing the output. Stream input is provided by low-level events; stream output is accomplished through low-level graphics.
Common Lisp Interface Manager 2.0 User's Guide - 20 Sep 2011