[lkb] filter rules for semantically empty elements

Hi!

I am doing my master's thesis on machine translation between Swedish and 
English using the lkb system.

In my grammars I have some semantically empty elements which makes 
generation very slow. The lkb also warns me about having elements with 
no semantics and no filter rules. My problem is that I don't know how to 
write these filter rules and I haven't been able to find any 
documentation about it anywhere. In the user manual it does say that it 
is possible to load heuristics for on the generate-menue, but there is 
no infomration on how these heuristics should be written.

Does anyone know where to find information about how to write filter 
rules/heuristics?

/Sara Stymne