[Help-glpk] Different constraint for first element of a set than for other elements

[Marc Goetschalckx]

The model has a different constraint for the first element of a set (say PERIODS) than for all remaining elements.
I have defined three sets:
INITIAL_PERIOD;
OTHER_PERIODS;
PERIODS := INITIAL_PERIOD union OTHER_PERIODS;
when MathProg is generating the model and specifically constraints over the PERIODS set it terminates with the following error

cannot convert JAN to floating-point number

where JAN is the first period in OTHER_PERIODS
How should the model be written in MathProg to allow different constraints for the first element.
Thanks