[Help-glpk] Index i = -M, 0 ~~~(Decrementing) how to implement in this MathProg

[Noli.Sicad]

I have a problem, i.e. constraint that requires index i decrementing
from -M to O, instead of usual incrementing by 1. How do AMPL/MathProg
decrement? How do we implement this in MathProg?

~~~~~
It goes this way (This is not a ampl/mathprog syntax)

Subject to Area:
 Summation (subscript k) in Xij + Win = Ai, i= -M,0 

Where: Ai = number of hectares present in period 1 that were regenerated
in period i, i= -M, 0 (hectares in each age-class).
~~~~~~

Thanks in advance.

Regards, Noli