[rules-users] counting facts of a kind

As simple as it gets: rule countHigh when $n : Number() from accumulate ( $sgc : StartGoalConn() count( $sgc ) ) then System.out.println( "plain routes: " + $n ); end And yet, it fails: Exception in thread "main" org.drools.RuntimeDroolsException: java.lang.ClassCastException: rss.ixl.route.TrainStartGoalConn cannot be cast to java.io.Serializable Sure, adding "implements Serializable" to StartGoalConn fixes this. But why is this necessary? FYI, StartGoalConn is abstract superclass of TrainStartGoalConn - just in case this has some bearing on this issue. Best -W