Here is what I thought of, but I am unsure if it is advisable.

Basically, I introduce a new variable "F" ("F" for function) and a new equation. In that equation, I set F equal to whatever formula I want it to equal. Then I can reuse this expression elsewhere.

Is this the way to do it?

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Here's an example. If I have a variable x(i,j) and I want to calculate the sum over i for fixed j, then I would write

**someequation(j) .. sum(i, x(i,j)) =e= 1 ;**

However, I could introduce a new variable F(j), and then I could write

**someequation(j) .. F(j) =e= 1;**

anotherequation(j) .. F(j) =e= sum(i, x(i,j)) ;

anotherequation(j) .. F(j) =e= sum(i, x(i,j)) ;

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Is this advisable or is there a better way? Note that if I needed to create another function involving summing x(i,j), I could just write F(j) in place of sum(i, x(i,j)). But is there not a built-in way to do this?