I am quite new to forum and a beginner to GAMS.

I want to form the equation attached. https://image.ibb.co/c41Qpb/equation.png

I already introduced the i,j and k to the software however when i add the order of (k-1) at the LHS of the equation it gives

**Error 170 Domain violation for element**when i want to introduce the equation. What should i do to manipulate the k index as i want?

I tried these two:

- function(i,j,k) .. x(i,j,'k-1')-x(i,j,k)=l=s(i,j,k)

- function(i,j,'k-1',k) .. x(i,j,'k-1')-x(i,j,k)=l=s(i,j,k)