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Changes between Initial Version and Version 1 of Ticket #1173, comment 9

initial	v1
1	1	The reason the original version doesn't work seems to be because the weak table's displacement code might run out of space, and simply not insert the symbol in the table.
2	2
3		The fixed version dispenses with the weak table, and instead changes buckets to weakly refer to their symbol. After a GC, it scans the symbol table, and updates the weak references if their targets are forwarded. If the result isn't in the new heap or permanent, it can be discarded.
	3	The fixed version dispenses with the weak table, and instead changes buckets to weakly refer to their symbol (by using `C_SPECIALBLOCK_BIT`). After a GC, it scans the symbol table, and updates the weak references if their targets are forwarded. If the result isn't in the new heap or permanent, it can be discarded.
4	4
5	5	This implementation is rather slow, because I had to move the scanning of the symbol table to be done even for minor GCs, because symbols can be purely stack-allocated and unreferenced by anything except the symbol table, yet still need to be preserved: this happens when the symbol gets a global definition or a property list.