Einstein Summation Convention

[Amine Aboufirass]

I would like for sympy to simplify the following expression:

Meaning I would like to explicitly see the summation and differentiation applied such that I can substitute in values for tau_ij and see how they affect the equation.

I only found one StackExchange post regarding summation convention in sympy (https://stackoverflow.com/questions/37647370/expand-index-notation-equation-using-sympy). The solution offered in this stackexchange post really shows how limited sympy is in this regard. I also found a package built on sympy that specializes in working with einstein summation convention https://github.com/LindaLinsefors/EinSumConv but it seems like this was begun 3 years ago and discontinued. 

What options exist for users who regularly need to expand expressions using the summation convention? I feel like this should be a part of any symbolic math package...