Tcl Thread extension.

You can use this extension to gain script-level access to Tcl
threading capabilities.

The extension can be used with Tcl cores starting from Tcl8.4 and
later.  Also, this extension supports, i.e. can be used as a loadable
module of, AOLserver 4.x series of the highly-scalable web server from
America Online.

You need to have your Tcl core compiled with "--enable-threads".