Conditionally define an equation

The conditions

Eq1$(qq gt 0)..    xvar=e=3;

Eq2$(sum(I,q(i)) gt 0)..  yvar=l=4;

Eq3(i)$(a(i) gt 0)..   ivar(i)=g= -a(i);

each cause an equation to exist in a model only if the condition is satisfied.

For more on this class of conditionals see the discussion in the Conditionals chapter.