code: func (p *Pipe) One(result interface{}) error {
  	iter := p.Iter()
  	if iter.Next(result) {
  		return nil
  	}
  	if err := iter.Err(); err != nil {
  		return err
  	}
  	return ErrNotFound
  }
docs: // One executes the pipeline and unmarshals the first item from the
  // result set into the result parameter.
  // It returns ErrNotFound if no items are generated by the pipeline.