Singleton in SNIT(SNIT's Not Incr Tcl)

在 SNIT 中要怎麼實作 Singleton pattern 呢？可以參考這段程式：

package require snit

snit::type Singleton {
    typevariable _instance {}
    
    typeconstructor {
        set _instance [Singleton %AUTO%]
    }
    
    constructor {args} {
        if {$_instance != {}} {
            error "Singleton can only be accessed through \"getInstance\""
        }
    }
    
    typemethod getInstance {} { 
        return $_instance
    }
}