trace add variable @head_list write [cb _write_head_list]
trace add variable @check_head write [cb _write_check_head]
grid $w.validate.head_r $w.validate.head_m -sticky w
trace add variable @head_list write [cb _write_head_list]
trace add variable @check_head write [cb _write_check_head]
grid $w.validate.head_r $w.validate.head_m -sticky w