[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.