GAMS: execute_unload

It would be useful to be able to write to the same GDX file in different steps. I.e. something like:

execute_unload "a.gdx",a;

a(i) = a(i)/1000;
b(i) = c(i) + a(i);

execute_unload "a.gdx","append",b; 

Currently we cannot do this: there is no facility to append.