cloud.google.com/go/pubsub iterator.go：44：未定义：acker和puller

I'm hitting the following error messages when I build codes that employ cloud.google.com/go/pubsub. Am I working on a wrong package? Or it's just because of alpha?

$ go test
# cloud.google.com/go/pubsub
./iterator.go:44: undefined: acker
./iterator.go:46: undefined: puller
FAIL    cloud.google.com/go/pubsub [build failed]