@tomleslie Thank you very much for your answer. Yes, using add will do the job. This was a toy model of my real computation. But I do not need a closed formula for the summation. So your proposal will work for me.

Your approach will do either. Now I have several choices to solve the problem. Thank you very much.

Thank you very much. If I decide to use alias (which seems to be risky), both your suggestions will do.

The reason I chose alias was I had to solve vector partial differenctial equations. What I showed was the easiest case where I had the same problem I was finding when working on the real system (more functions and more variables). Best regards.

Thank you very much for your answer. Now it is clear to me what alias was doing. Regards.