Checking for variables with a specific index, whether the value is equal to a certain value.

Hello, I have a problem.

I created two sets i and j for the variable X_i,j.

Now I want to add a constraint to see if the sum of all X_i,j with i="specific value” is equal to 10.

My guess would be something like:

sum[ ("Specific value”,j), variable(i,j) ] = 10;

But this obviously doesn't work, otherwise I wouldn't be asking.