Equation term consideration limited using conditions

(transbad.gms vs transfix.gms)

An equation such as

slDEMANDEQ(MARKET)..

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

can be defined and cause the problem to contain less variables and equations thus using less memory by adding conditionals both on equation and term existence to avoid considering unnecessary cases

fsDEMANDEQ(MARKET)$demand(market)..

  SUM(PLANT $cost(plant,market),SHIP(PLANT, MARKET))

    =G=

  DEMAND(MARKET);