Question: How to take derivative of sum?


I have the following expression.


What I want to do is take the derivative of this expression with respect to,


Where L is somewhere between 1 and n. 

Using the diff operator, I would like to get something like,


But I don't.  Instead, I get zero.  I want to leave n undefined so that the summation isn't automatically expanded out to alot of terms, thereby making it much more difficult to read.  This is especially true as my formulas become more complicated.




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