Godeps/_workspace/src/gopkg.in/mgo.v2/session.go
func (p *Pipe) Explain(result interface{}) error {
	c := p.collection
	cmd := pipeCmd{
		Aggregate: c.Name,
		Pipeline:  p.pipeline,
		AllowDisk: p.allowDisk,
		Explain:   true,
	}
	return c.Database.Run(cmd, result)
}
func (p *Pipe) Explain(result interface{}) error {
	c := p.collection
	cmd := pipeCmd{
		Aggregate: c.Name,
		Pipeline:  p.pipeline,
		AllowDisk: p.allowDisk,
		Explain:   true,
	}
	return c.Database.Run(cmd, result)
}
// Explain returns a number of details about how the MongoDB server would
// execute the requested pipeline, such as the number of objects examined,
// the number of times the read lock was yielded to allow writes to go in,
// and so on.
//
// For example:
//
//     var m bson.M
//     err := collection.Pipe(pipeline).Explain(&m)
//     if err == nil {
//         fmt.Printf("Explain: %#v\n", m)
//     }
//