Without any conditions and constraints, this is just a simple computation you could code in Java and invoke in some appropriate place. But I suspect you haven't told us the whole story... You don't say where this "list" is; so I assume it's a field in some other fact. This code is absolutely untested. rule count when ListHolder( $list : list ) $map: Map from accumulate ( XXX( $id: id, $type: type ) from $list, init( Map map = new HashMap(); ), action( Integer count = map.get( $type ); count = Integer.valueOf( count == null ? 1 : count+1 ); map.put( $type, count ); ), result( map ) ) then // ... use $map end -W 2010/11/5 Yaniv Itzhaki <iyaniv(a)gmail.com&gt;