In my equation definitions, I need to use certain expressions involving the variables over and over again. How can I define a function that I can reuse in place of writing the expressions?

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?

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)) ;**

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?