code: func (f *RangeFilter) MarshalJSON() ([]byte, error) {
  	root := map[string]map[string]map[string]interface{}{
  		"range": {
  			f.Key: {
  				"lte": f.Lte,
  				"gte": f.Gte,
  			},
  		},
  	}
  
  	if f.Format != "" {
  		root["range"][f.Key]["format"] = f.Format
  	}
  
  	return json.Marshal(root)
  }
docs: // MarshalJSON returns the JSON encoding of the query string filter.