You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: manifold.go
    path of file: ./repos/juju/worker/gate
    
    the code of the file until where you have to start completion: // Copyright 2015 Canonical Ltd.
// Licensed under the AGPLv3, see LICENCE file for details.

package gate

import (
	"sync"

	"github.com/juju/errors"
	"github.com/juju/worker/v3"
	"github.com/juju/worker/v3/dependency"
	"gopkg.in/tomb.v2"
)

// Manifold returns a dependency.Manifold that wraps a single channel, shared
// across all workers returned by the start func; it can be used to synchronize
// operations across manifolds that lack direct dependency relationships.
//
// The output func accepts an out pointer to either an Unlocker or a Waiter.
func Manifold() dependency.Manifold {
	return ManifoldEx(nil)
}

// ManifoldEx does the same thing as Manifold bu
    
func ManifoldEx(lock Lock) dependency.Manifold {
	return dependency.Manifold{
		Start: func(_ dependency.Context) (worker.Worker, error) {
			// Need to assign a copy of the arg so we don't
			// modify the variable in the closure when we get
			// called a second time.
			wLock := lock
			if wLock == nil {
				wLock = NewLock()
			}
			w := &gate{lock: wLock}
			w.tomb.Go(func() error {
				<-w.tomb.Dying()
				return nil
			})
			return w, nil
		},
		Output: func(in worker.Worker, out interface{}) error {
			inWorker, _ := in.(*gate)
			if inWorker == nil {
				return errors.Errorf("in should be a *gate; is %#v", in)
			}
			switch outPointer := out.(type) {
			case *Unlocker:
				*outPointer = inWorker.lock
			case *Waiter:
				*outPointer = inWorker.lock
			case *Lock:
				*outPointer = inWorker.lock
			default:
				return errors.Errorf("out should be a *Unlocker, *Waiter, *Lock; is %#v", out)
			}
			return nil
		},
	}
}