code: func addMissingFields(dest *bigquery.Schema, src bigquery.Schema) {
  	destFields := indexFields(*dest)
  	for _, sf := range src {
  		switch df := destFields[sf.Name]; {
  		case df == nil:
  			*dest = append(*dest, sf)
  		default:
  			addMissingFields(&df.Schema, sf.Schema)
  		}
  	}
  }
docs: // addMissingFields copies fields from src to dest if they are not present in
  // dest.