As promised yesterday I added a cells-store facility to cells. The code can be found at the bottom of md-utilities in CVS. Basically, this is a cells-aware hash table with some syntactic sugar. Here's part of the test function included in md-utilities: (defun test-cells-store () (trc "testing cells-store -- making objects") (let* ((store (make-instance 'cells-store)) (foo (make-instance 'test-store-item :value (c?-with-stored (v :foo store 'nothing) (bwhen (val (value v)) val)))) (foo+1 (make-instance 'test-store-item :value (c?-with-stored (v :foo store 'nothing) (bwhen (val (value v)) (1+ val)))))) (store-add :foo store (make-instance 'family :value (c-in nil))) (setf (value (store-lookup :foo store)) 1) (store-remove :foo store)) So you create a ruled cell with c?-with-stored. This can happen before or after key is added to the cells-store. When the rule is the executed, when an object is added or removed from key in the hash table *or* when any of the cells on the object change. The rule is not executed, if other stuff in the hash table changes. The (c?-with-stored (var key store &optional default) &body body) macro works like bwhen -- that is, if key is present, body is executed with var bound to the object identified by key -- and if not, default is returned. That means, body can assume that var is bound to a valid object (like in the example, it is safe to call value on it, if you know that you only store family instances in the store) Calling store-remove will reset all dependent cells to their default. The change is propagated as soon as store-add, setf, or store-remove are called -- meaning that observers will do the expected thing. Maybe someone will find it useful. Peter