A boundary in Alexander's sense is where two light cones meet, defining separation and presence between spaces.

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  "claim": "A boundary in Alexander's sense is where two light cones meet, defining separation and presence between spaces.",
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  "section": "6. The 15 Properties — opened, not enumerated",
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  "supporting_refs": [
    "Christopher Alexander",
    "Boundaries property",
    "Levin light cones"
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