private boolean isRootOperation(BuildFrame frame) {
        return frame.astNode == root && frame.state.groupBy == null;
    }
Constructs a set of Vespa specific grouping request that corresponds to the parameters given to this builder.
This method might fail due to unsupported constructs in the request, in which case an exception is thrown.

@throws IllegalStateException         If this method is called more than once.
@throws IllegalInputException If the grouping request contains unsupported constructs.
container-search/src/main/java/com/yahoo/search/grouping/vespa/RequestBuilder.java