SET(WIDGET_CALLBACK)

  Specifies the TPU program or learn sequence to be called by TPU when
  a widget callback occurs for the widget instance.

  Syntax

     SET (WIDGET_CALLBACK, widget, program_source, closure)

  Parameters

     WIDGET_CALLBACK      A keyword directing TPU to set the
                          application-level widget callback.

     widget               The widget instance whose callback you want to
                          set.

     program_source       The string, buffer, range, learn sequence, or
                          program specifying the application-level callback.
                          This code is executed when the widget performs a
                          callback to TPU.

     closure              A string or integer.  TPU passes the value to
                          the application when the widget performs a
                          callback to TPU.

  Example

     The following statement designates the procedure "user_scroll_dispatch"
     as the callback routine handling events from the scroll bar widget.
     The statement designates the closure for the callback as the character
     "h".

     SET (WIDGET_CALLBACK, scroll_bar_widget, "USER_SCROLL_DISPATCH", "h");