[incr Tcl] is an object system for the Tcl programming language.

[incr Tcl] provides the extra language support needed to build large Tcl/Tk
applications. It introduces the notion of objects, which act as building
blocks for an application. Each object is a bag of data with a set of
procedures or "methods" that are used to manipulate it. Objects are
organized into "classes" with identical characteristics, and classes can
inherit functionality from one another. This object-oriented paradigm adds
another level of organization on top of the basic variable/procedure
elements, and the resulting code is easier to understand and maintain.

The Tcl package of Slackware 14.1 contains itcl-4.0.0.

This package contains the older itcl-3.4.0 which may be required by some
programs like mcu8051ide. This package may be installed in a Slackware 14.1
system without conflicting with the Slackware package.
