proc check_myOS {} {
  global tcl_platform
  set wa 3
  set w1 [string range [array get tcl_platform os] $wa $wa]
  if { $w1 == "\{" } { set wa [expr $wa + 1] }
  set re [string range [array get tcl_platform os] $wa [expr $wa + 1]]
  if { $re == "Da" } { return {mac} }
  if { $re == "Wi" } { return {win} }
  return $re
}


set w1 [check_myOS]

button .b1 -text $w1 -command "exit"
pack .b1