func (conn *Conn) NewSubscriptionSet() *SubscriptionSet {
	return &SubscriptionSet{newSet(), conn}
}
NewSubscriptionSet returns a new subscription set.
package dbus
import (
	"time"
)
type SubscriptionSet struct {
	*set
	conn *Conn
}
func (s *SubscriptionSet) filter(unit string) bool {
	return !s.Contains(unit)
}
func (s *SubscriptionSet) Subscribe() (<-chan map[string]*UnitStatus, <-chan error) {
	return s.conn.SubscribeUnitsCustom(time.Second, 0,
		mismatchUnitStatus,
		func(unit string) bool { return s.filter(unit) },
	)
}