Various improvements to `POSet` #3686

Job	Run time
Version Sync	7s
Nix flake (self-hosted, linux, normal)	9m 59s
Nix flake (macos-13, macos-13)	17m 37s
Nix flake (MacM1, self-macos-12)	18s
Check Java code formatting	19s
K Ubuntu Jammy Package	25m 5s
K Tests	1h 9m 43s
Performace Tests	15m 50s
	2h 18m 58s

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