Extended Tcl (TclX), is a set of extensions to Tcl, the Tool
Command Language invented by Dr. John Ousterhout of the University
of California at Berkeley.  Tcl is a powerful, yet simple embeddable
programming language.  Extended Tcl is oriented towards Unix system
programming tasks, with many additional interfaces to the Unix
operating system, It is upwardly compatible with Tcl.  You take
the Extended Tcl package, add it to Tcl, and from that you get
Extended Tcl.  (Berkeley Tcl is not included in this distribution,
obtain it from ftp.cs.berkeley.edu).