untap(l) {
        const ix = this.listeners.indexOf(l);
        if (ix == -1) {
            return;
        }

        // Make a copy of the listeners to avoid the all to common
        // unsubscribe-during-dispatch problem
        this.listeners = this.listeners.slice(0);
        this.listeners.splice(ix, 1);
    }
Remove the listener, l, from the active listeners of this signal. If
l is not currently tapped into this signal, this method will complete
successfully without changing the collection of listeners.

@param {function} l