trace add variable @check_head write [cb _write_check_head]
grid $w.validate.head_r $w.validate.head_m -sticky w
radiobutton $w.validate.always_r \
trace add variable @check_head write [cb _write_check_head]
grid $w.validate.head_r $w.validate.head_m -sticky w
radiobutton $w.validate.always_r \