[Help-glpk] Modelling differences in time

[Jiri K]

Dear all,
I would appreciate a hint for the following problem:
In my model I need to care about the values of a variable in consecutive
periods. Therefore in the formulas I operate with differences like 

{i in PERIOD: i>1} x[i]-x[i-1]

However this only works when periods are described with numbers which always
increment by 1. How could I simply model this if the members of the PERIOD
set are e.g. 'Jan', 'Feb' etc?

Thank you for your answer

Jiri
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