diff --git a/issue-83-cook-membrane.png b/issue-83-cook-membrane.png
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diff --git a/issue-83-cook-membrane.svg b/issue-83-cook-membrane.svg
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--- /dev/null
+++ b/issue-83-cook-membrane.svg
@@ -0,0 +1,58 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="1000" height="900" viewBox="0 0 1000 900" font-family="Helvetica, Arial, sans-serif">
+  <rect width="1000" height="900" fill="#ffffff"/>
+  <!-- scale: 10 px per mm; origin (0,0) at (250, 760); y up -->
+  <defs>
+    <marker id="arr" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="8" markerHeight="8" orient="auto-start-reverse">
+      <path d="M0,0 L10,5 L0,10 z" fill="#1f4e9c"/>
+    </marker>
+    <marker id="dim" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse">
+      <path d="M0,0 L10,5 L0,10 z" fill="#444"/>
+    </marker>
+    <pattern id="hatch" width="10" height="10" patternUnits="userSpaceOnUse" patternTransform="rotate(45)">
+      <line x1="0" y1="0" x2="0" y2="10" stroke="#555" stroke-width="2"/>
+    </pattern>
+  </defs>
+  <!-- clamp -->
+  <rect x="220" y="300" width="30" height="460" fill="url(#hatch)" stroke="none"/>
+  <line x1="250" y1="290" x2="250" y2="770" stroke="#222" stroke-width="3"/>
+  <!-- panel: (0,0)->(250,760); (48,44)->(730,320); (48,60)->(730,160); (0,44)->(250,320) -->
+  <polygon points="250,760 730,320 730,160 250,320" fill="#dbe7f7" stroke="#1b2a41" stroke-width="3"/>
+  <!-- shear load on right edge -->
+  <g stroke="#1f4e9c" stroke-width="3" marker-end="url(#arr)">
+    <line x1="770" y1="330" x2="770" y2="175"/>
+    <line x1="800" y1="330" x2="800" y2="175"/>
+    <line x1="830" y1="330" x2="830" y2="175"/>
+  </g>
+  <line x1="730" y1="160" x2="845" y2="160" stroke="#1f4e9c" stroke-width="1.5" stroke-dasharray="4 4"/>
+  <line x1="730" y1="320" x2="845" y2="320" stroke="#1f4e9c" stroke-width="1.5" stroke-dasharray="4 4"/>
+  <text x="745" y="380" font-size="26" fill="#1f4e9c">F (total,</text>
+  <text x="745" y="410" font-size="26" fill="#1f4e9c">uniform shear)</text>
+  <!-- point C -->
+  <circle cx="730" cy="160" r="8" fill="#c0392b"/>
+  <text x="700" y="140" font-size="28" fill="#c0392b" font-weight="bold">C (48, 60)</text>
+  <!-- corner labels -->
+  <text x="262" y="790" font-size="22" fill="#333">(0, 0)</text>
+  <text x="262" y="345" font-size="22" fill="#333">(0, 44)</text>
+  <text x="625" y="306" font-size="22" fill="#333">(48, 44)</text>
+  <!-- dimensions -->
+  <g stroke="#444" stroke-width="1.5" marker-start="url(#dim)" marker-end="url(#dim)">
+    <line x1="250" y1="840" x2="730" y2="840"/>
+    <line x1="150" y1="760" x2="150" y2="320"/>
+    <line x1="940" y1="320" x2="940" y2="160"/>
+    <line x1="80" y1="760" x2="80" y2="160"/>
+  </g>
+  <g stroke="#999" stroke-width="1" stroke-dasharray="3 4">
+    <line x1="730" y1="330" x2="730" y2="850"/>
+    <line x1="60" y1="160" x2="730" y2="160"/>
+    <line x1="140" y1="320" x2="250" y2="320"/>
+    <line x1="60" y1="760" x2="250" y2="760"/>
+    <line x1="845" y1="160" x2="950" y2="160"/>
+    <line x1="845" y1="320" x2="950" y2="320"/>
+  </g>
+  <text x="470" y="830" font-size="24" fill="#222">48</text>
+  <text x="112" y="548" font-size="24" fill="#222">44</text>
+  <text x="950" y="248" font-size="24" fill="#222">16</text>
+  <text x="38" y="470" font-size="24" fill="#222">60</text>
+  <text x="290" y="60" font-size="30" fill="#111" font-weight="bold">Cook's membrane (lengths in mm)</text>
+  <text x="290" y="96" font-size="22" fill="#444">clamped on x = 0; vertical shear on x = 48; plane strain, unit thickness</text>
+</svg>
diff --git a/issue-85-bracket-octree.png b/issue-85-bracket-octree.png
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diff --git a/issue-85-plate-octree.png b/issue-85-plate-octree.png
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