Add neutral triangle definition and use up/down/neutral triangle terms in lemma

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

papers/plane_depth_sequencing/paper.log

@@ -1,4 +1,4 @@
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@@ -178,11 +178,11 @@ L3 programming layer <2025-06-09>
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-Output written on paper.pdf (2 pages, 143149 bytes).
+Output written on paper.pdf (2 pages, 144144 bytes).
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