Purpose: discuss for the beginner the ''types'' of variables a user can
use in a Tcl script.

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   * [[Discuss characters permitted in a variable name, format of a variable name, when to use the $ and when not, etc.]]

   * Types of variables

   * In the core of Tcl

   * Scalar
   1 String
   1 List
   1 Handle
   1 Namespace
   1 Thread

   * [Array]

   * [Magic Name]

   * In common extensions

   * In Tk

   * widget
   * font

   * In TclX

   * keyed list