public void addGroupingResult(Grouping executionResult) {
        executionResult.unifyNull();
        rootBuilder.addGroup(executionResult.getRoot());
    }
Adds a grouping result to this transform. This method will recurse through the given object and retrieve all the
information it needs to produce the desired result when calling {@link #build()}.

@param executionResult the grouping result to process
container-search/src/main/java/com/yahoo/search/grouping/vespa/ResultBuilder.java