diff --git a/img/rwx.svg b/img/rwx.svg
new file mode 100644
index 0000000..7a2a9ee
--- /dev/null
+++ b/img/rwx.svg
@@ -0,0 +1,54 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN"
+ "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
+<!-- Generated by graphviz version 2.43.0 (0)
+ -->
+<!-- Title: index Pages: 1 -->
+<svg width="118pt" height="119pt"
+ viewBox="0.00 0.00 118.50 119.00" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+<g id="graph0" class="graph" transform="scale(1 1) rotate(0) translate(4 115)">
+<title>index</title>
+<!-- x -->
+<g id="node1" class="node">
+<title>x</title>
+<ellipse fill="#303030" stroke="#00a000" stroke-width="2" cx="54" cy="-93" rx="18" ry="18"/>
+<text text-anchor="middle" x="54" y="-89.3" font-family="DejaVu Sans" font-size="14.00" fill="#ffffff">X</text>
+</g>
+<!-- r -->
+<g id="node2" class="node">
+<title>r</title>
+<ellipse fill="#303030" stroke="#d00000" stroke-width="2" cx="18" cy="-19.5" rx="18" ry="18"/>
+<text text-anchor="middle" x="18" y="-15.8" font-family="DejaVu Sans" font-size="14.00" fill="#ffffff">R</text>
+</g>
+<!-- x&#45;&gt;r -->
+<g id="edge4" class="edge">
+<title>x&#45;&gt;r</title>
+<path fill="none" stroke="#00a000" stroke-width="2" d="M46.19,-76.48C41.57,-67.3 35.61,-55.46 30.36,-45.04"/>
+<polygon fill="#00a000" stroke="#00a000" stroke-width="2" points="33.42,-43.34 25.8,-35.98 27.17,-46.49 33.42,-43.34"/>
+</g>
+<!-- w -->
+<g id="node3" class="node">
+<title>w</title>
+<ellipse fill="#303030" stroke="#0000ff" stroke-width="2" cx="91" cy="-19.5" rx="19.5" ry="19.5"/>
+<text text-anchor="middle" x="91" y="-15.8" font-family="DejaVu Sans" font-size="14.00" fill="#ffffff">W</text>
+</g>
+<!-- r&#45;&gt;w -->
+<g id="edge1" class="edge">
+<title>r&#45;&gt;w</title>
+<path fill="none" stroke="#d00000" stroke-width="2" d="M33.68,-10.39C43.86,-6.09 54.04,-4.95 64.22,-6.96"/>
+<polygon fill="#d00000" stroke="#d00000" stroke-width="2" points="63.4,-10.36 73.98,-9.85 65.39,-3.65 63.4,-10.36"/>
+</g>
+<!-- w&#45;&gt;x -->
+<g id="edge3" class="edge">
+<title>w&#45;&gt;x</title>
+<path fill="none" stroke="#0000ff" stroke-width="2" d="M82.34,-37.23C77.69,-46.22 71.86,-57.48 66.71,-67.44"/>
+<polygon fill="#0000ff" stroke="#0000ff" stroke-width="2" points="63.52,-65.99 62.03,-76.48 69.73,-69.21 63.52,-65.99"/>
+</g>
+<!-- w&#45;&gt;r -->
+<g id="edge2" class="edge">
+<title>w&#45;&gt;r</title>
+<path fill="none" stroke="#0000ff" stroke-width="2" d="M73.98,-29.15C63.8,-33.17 53.62,-34.04 43.44,-31.76"/>
+<polygon fill="#0000ff" stroke="#0000ff" stroke-width="2" points="44.28,-28.35 33.68,-28.61 42.12,-35.01 44.28,-28.35"/>
+</g>
+</g>
+</svg>