Hand-applied mutants of CultElementIds.cs lines 163/167/180 (Append to Prepend), 248 (union cycle guard, stack overflow), 322 (value list Holds), 583 (KeyOf inherit), 629-631 and 636 (IsWrittenWhole/IsKeyed inherit and logic) now fail these tests.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>