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Pol097 is now pol096 with new feature or a complete different core?
I can think pol097 is stable like pol096 except for problems in new feature or I can expect it is more unstable because internal loop and cores are completely rewrittten?

Pol097 is changed also inside?

POL097 is POL096 with more features being added but also things being shifted around and removed. UO.em was split up and many old unused things are being removed. Basically 097 is pretty stable right now because nothing so extremely major that it would threaten stability has been done. But with many things in their roadmap to 097's final version changing many vital functions in POL, it might become unstable at some point before going final.

POL 097 has actually had a lot of code rewritten, but we did a lot of testing before the first public release. There were a few crashes prior to it.
I removed a few thousand lines of code from the core when I optimized member and method access ... which is still a big thing to test, to make sure I got everything right, it was as Ron Burgandy would say "kind of a big deal".
Its what took POL 097 to 2.07 megs from POL 096's 2.15 megs.

The skill usage system was also rewritten and made a lot smaller ( how it uses attributes.cfg and starts skill scripts when that skill gump is used ).

So theres similarities, and important differences too.