public RequestBuilder setRootOperation(GroupingOperation root) {
        Objects.requireNonNull(root, "Root must be non-null");
        this.root = root;
        return this;
    }
Sets the abstract syntax tree of the request whose back-end queries to create.

@param root The grouping request to convert.
@return This, to allow chaining.
container-search/src/main/java/com/yahoo/search/grouping/vespa/RequestBuilder.java