code: func (b *CollapseBuilder) Source() (interface{}, error) {
  	// {
  	//   "field": "user",
  	//   "inner_hits": {
  	//     "name": "last_tweets",
  	//     "size": 5,
  	//     "sort": [{ "date": "asc" }]
  	//   },
  	//   "max_concurrent_group_searches": 4
  	// }
  	src := map[string]interface{}{
  		"field": b.field,
  	}
  
  	if b.innerHit != nil {
  		hits, err := b.innerHit.Source()
  		if err != nil {
  			return nil, err
  		}
  		src["inner_hits"] = hits
  	}
  
  	if b.maxConcurrentGroupRequests != nil {
  		src["max_concurrent_group_searches"] = *b.maxConcurrentGroupRequests
  	}
  
  	return src, nil
  }
docs: // Source generates the JSON serializable fragment for the CollapseBuilder.