Equation existence using conditionals

(transbad.gms vs transfix.gms)

Equations such as

slDEMANDEQ(MARKET)..

SUM(PLANT, SHIP(PLANT, MARKET))=G=DEMAND(MARKET);

may not really be relevant for all possible elements of the MARKET set and their uncontrolled generation involves GAMS needing to deal with more equations thus using more memory.  I might only define equations when there is nonzero demand.  Program execution memory use and model size would be reduced by

inDEMANDEQ(MARKET)$demand(market)..

SUM(PLANT,  SHIP(PLANT, MARKET)) =G= DEMAND(MARKET);