add support for removing an item from a reference
add a facility of flush the cache in a predicate
minor debug message change
add a list of dependent machines to the describe output
when listening to a new machine, flush the predicate caches
add a method to remove dependancies
add a method to remove local variables
do not raise errors when machines try to refer to the ITEM in a reference when the reference is not assigned
reverting previous change, null is now back to being a specific value
clean up setting of dependencies when items are removed from references
cleanup the setup of dependencies when an item is assigned to a reference
started to add code (disabled) that will permit properties to store item references being added to a list