There are many fundamental decisions which can't really be pluggable. Can the GC move? (Then you must be careful storing pointers if a GC can occur). Is it the

There are many fundamental decisions which can't really be pluggable. Can the GC move? (Then you must be careful storing pointers if a GC can occur). Is it the user's responsibility to note when an object changes? (Which can be faster, but needs lots of bookkeeping code). Are objects kept alive by internal pointers, or only pointers to their start?