Other difficult problems 

* Complementary properties 
   Parent(X,Y) <=> Child(Y,X) <=> Nonempty(X) 
   
* Multi-step operations 
   { Perform(action); Report(action) } 
   
* Adding steps 
   do Message as { "Prologue"; Message } 
   do Message as { Message; "Epilogue" } 
   
* Retaining information across rules