@Override
    public GroupingOperation setLabel(String label) {
        super.setLabel(label);
        return this;
    }
Recursively calls {@link GroupingExpression#visit(ExpressionVisitor)} on all {@link GroupingExpression} objects
in this operation and in all of its child operations.

@param visitor The visitor to call.
container-search/src/main/java/com/yahoo/search/grouping/request/GroupingOperation.java