if 0 {
 experiment to make tcl understand 'a = c' mathematical commands.
 Starts with a standard command 'realnumber' and its Cmd, cget and configure scripts
 then a test script exercises it to demonstrate its 'use'.
}

  proc realnumber {w args} { ;# a variable
	global $w.props ;# an array of options specific to the dimValue 'class'
	# define option list and each default value
	array set $w.props {-value 0 }
	
	set textArgs {} ;# list of arguments not specific to the class
	foreach {opt val} $args {
		if {[array names $w.props $opt]!=""} {set options($opt) $val
		} else { lappend textArgs $opt $val }
	}

	eval interp create $w ;# create the "procedure" w

	interp hide {} $w
	# Install the alias:
   # realnumberCmd are sub-commands for realnumber class
	interp alias {} $w {} realnumberCmd $w
	foreach opt [array names options] {
		$w configure $opt $options($opt)
	}
  	return $w ;# the original object
  }
  proc realnumberCmd {self cmd args} {
	switch -- $cmd {
		configure {eval realnumberConfigure $self $cmd $args}
		cget {eval realnumberCget $self $args}
		"=" { $self configure -value $args }
		"?" { return [$self cget -value] }
	}
  }
  proc realnumberConfigure {self cmd args} {
	# 3 cases:
	#
	# $args is empty       -> return all options with their values
	# $args is one element -> return current values
	# $args is 2+ elements -> configure the options
	#puts "Config comd $self $cmd $args [llength $args]"
	global $self.props
	switch [llength $args] {
		0 { ;# return all options 
			set result [array names $self.props]
			return $result
		}
		1 { ;# return argument values
			lappend opts [$self cget $args]
			return $opts
		}
		default { ;# >1 arg - an option and its value
			foreach {option value} $args { ;# go through each option:
				if {[array names $self.props $option]=="-value"} {
					set $self.props($option) [expr $value]
				} elseif {[array names $self.props $option]!=""} {
	# access global array element for each added option. 
					set $self.props($option) $value
				} else { ;#  try the default $option, $value for $self
					$self configure $option $value
				}
			}
			return {}
		}
	}
  }
   proc realnumberCget {self args} {	;# cget defaults done by the interp cget command
	upvar #0 $self.props props ;# get local address for global array
	if {[array names props $args ]!=""} {
			return $props($args)
	}
	return 1 ;# [uplevel 1 [list interp invokehidden {} $self cget $args]]
  }

  proc test {} {
	realnumber biff
	set i 1
	while {$i<12} {
		set j 1
		while {$j<12} {
			biff = $i*$j
			puts "$i*$j is [biff ?]" ;# or use [biff cget -value]
			incr j
		}
		incr i
	}
	puts "factorials up to 14 NB greater than 16 this algorithm gets into trouble with integer overruns"
	set j 1
	biff = 1
	while {$j<14} {
		biff = [biff ?]*$j
		puts "$j! is [biff ?]" ;# or use [biff cget -value]
		incr j
	}
  }
  console show; update idletasks
  test