HI, There was an interesting discussion today on IRC; a user "jochen" reported NPEs when trying to iterate lists in the RHS constructed in the LHS using "from collect". I was wondering how Drools handles lists constructed in the LHS; if the list contained 5 elements but another rule retracted one of those elements would the list available in the RHS contain 4 or 5 elements? Here's a simplification of what was posted at http://pastebin.com/yKGSxFkA Rule 1 when $cheese : ArrayList() from collect(CheeseFact()); then for(int i=0; i<$cheese.size(); i++) { //Something useful... } end What if there was another rule that did something like this:- Rule 2 When $cf : CheeseFact(type == "chedder") Then retract($cf); end What would be the list Rule 1 iterates? I'd assume since I believe activation execution to be serial it would depend on the agenda's conflict resolution strategy or rule salience? What would be the learned opinion? With kind regards, Mike