public void visitExpressions(ExpressionVisitor visitor) {
        for (GroupingExpression exp : aliases.values()) {
            exp.visit(visitor);
        }
        for (GroupingExpression exp : outputs) {
            exp.visit(visitor);
        }
        for (GroupingExpression exp : orderBy) {
            exp.visit(visitor);
        }
        if (groupBy != null) {
            groupBy.visit(visitor);
        }
        for (GroupingOperation op : children) {
            op.visitExpressions(visitor);
        }
    }
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