How to get the next month in Period Member into Calculation Rule into FIX

Hello gurus,

I have the following business rule:

XRANGE(Ene, {Mes_Corte}))

Budget=Actual;

ENDFIX;

What we need is to assing a value for the next month selected into the variable Mes_Corte. For example if Mes_Corte="May":

FIX(FY15, {Mes_Corte}+1))     /* It has to be June */

Budget=1000;

ENDFIX;

Is there any function o proposal to use?

Thanks in advance