if 0 {[Richard Suchenwirth] 2003-03-12 - Here's a cute little application for the [iPAQ]: 
a signal light. Dirt simple: fill the display in a single color, controlled by the 
"cursor keys" (navigation button up/down/left/right). But when background lighting is on,
you can use this for signalling in the dark - or, with the default white, as a mild pocket
lamp... or to test whether the display's pixels are all working...
Yellow looks pretty beige, though. A cheap thrill at less than 900 bytes, this documentation 
included ;-)
}
 proc sigbind {w color} {
    if {[$w cget -background] != "white"} {set color white}
    $w config -background $color
 }
 pack [canvas .c -background white -height 320]
 wm geometry . +0+0
 bind . <Up>    {sigbind .c red}
 bind . <Down>  {sigbind .c green}
 bind . <Left>  {sigbind .c yellow}
 bind . <Right> {sigbind .c blue}
if 0 {
----
[Arts and crafts of Tcl-Tk programming]
}