In ExternalSemaphoreTable I noticed two class variables set by class initialization as: ProtectAdd := Semaphore forMutualExclusion. ProtectRemove := Semaphore forMutualExclusion and I'm curious, what are the TWO shared resources that these TWO are meant to protect? it seems there is only one, that being unprotectedExternalObjects. I notice that "ExternalSemaphoreTable class>>safelyUnregisterExternalObject:" stores unprotectedExternalObjects in its local variable "objects" and then up removes an object from it. But "ExternalSemaphoreTable class>>safelyRegisterExternalObject:" also stores unprotectedExternalObjects in its local variable "objects" which it then passes to "ExternalSemaphoreTable class>>collectionBasedOn:withRoomFor:" which takes a copy to grow a new collection to store into unprotectedExternalObjects. Even though its a rare case that the external semaphore might grow at just the same time that an object is being unregistered, and also practically #safelyUnregisterExternalObject: might not be preemptable, this smells bad having two mutual exclusions covering one resource - thus no mutual exclusion at all. Is there something I'm missing? cheers -ben