s needed to provide OID->instance mapping. Since placing object in hash increase its access counter, 
such object can not be deallocated by garbage collector. So after some time all persistent objects from 
the storage will be loaded to the memory. To solve the problem almost all languages with implicit
memory deallocation (garbage collection) provides weak references. But in some of them weak references are not supported
and sometime implementation of weak references is very inefficient. 
So to prevent mem