diff --git a/docs/img/arrow.svg b/docs/img/arrow.svg
new file mode 100644
index 00000000..782d119e
--- /dev/null
+++ b/docs/img/arrow.svg
@@ -0,0 +1,102 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<!-- Created with Inkscape (http://www.inkscape.org/) -->
+
+<svg
+   xmlns:dc="http://purl.org/dc/elements/1.1/"
+   xmlns:cc="http://creativecommons.org/ns#"
+   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+   xmlns:svg="http://www.w3.org/2000/svg"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+   width="120"
+   height="120"
+   id="svg2"
+   version="1.1"
+   inkscape:version="0.48.4 r9939"
+   sodipodi:docname="arrow.svg">
+  <defs
+     id="defs4">
+    <marker
+       inkscape:stockid="TriangleOutL"
+       orient="auto"
+       refY="0.0"
+       refX="0.0"
+       id="TriangleOutL"
+       style="overflow:visible">
+      <path
+         id="path4465"
+         d="M 5.77,0.0 L -2.88,5.0 L -2.88,-5.0 L 5.77,0.0 z "
+         style="fill-rule:evenodd;stroke:#000000;stroke-width:1.0pt"
+         transform="scale(0.8)" />
+    </marker>
+    <marker
+       inkscape:stockid="Arrow2Lend"
+       orient="auto"
+       refY="0.0"
+       refX="0.0"
+       id="Arrow2Lend"
+       style="overflow:visible;">
+      <path
+         id="path3788"
+         style="fill-rule:evenodd;stroke-width:0.62500000;stroke-linejoin:round;"
+         d="M 8.7185878,4.0337352 L -2.2072895,0.016013256 L 8.7185884,-4.0017078 C 6.9730900,-1.6296469 6.9831476,1.6157441 8.7185878,4.0337352 z "
+         transform="scale(1.1) rotate(180) translate(1,0)" />
+    </marker>
+    <marker
+       inkscape:stockid="Arrow1Lend"
+       orient="auto"
+       refY="0.0"
+       refX="0.0"
+       id="Arrow1Lend"
+       style="overflow:visible;">
+      <path
+         id="path3770"
+         d="M 0.0,0.0 L 5.0,-5.0 L -12.5,0.0 L 5.0,5.0 L 0.0,0.0 z "
+         style="fill-rule:evenodd;stroke:#000000;stroke-width:1.0pt;"
+         transform="scale(0.8) rotate(180) translate(12.5,0)" />
+    </marker>
+  </defs>
+  <sodipodi:namedview
+     id="base"
+     pagecolor="#ffffff"
+     bordercolor="#666666"
+     borderopacity="1.0"
+     inkscape:pageopacity="0.0"
+     inkscape:pageshadow="2"
+     inkscape:zoom="2.8"
+     inkscape:cx="-26.815683"
+     inkscape:cy="69.781644"
+     inkscape:document-units="px"
+     inkscape:current-layer="layer1"
+     showgrid="false"
+     inkscape:window-width="1920"
+     inkscape:window-height="1030"
+     inkscape:window-x="1920"
+     inkscape:window-y="0"
+     inkscape:window-maximized="1" />
+  <metadata
+     id="metadata7">
+    <rdf:RDF>
+      <cc:Work
+         rdf:about="">
+        <dc:format>image/svg+xml</dc:format>
+        <dc:type
+           rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
+        <dc:title />
+      </cc:Work>
+    </rdf:RDF>
+  </metadata>
+  <g
+     inkscape:label="Layer 1"
+     inkscape:groupmode="layer"
+     id="layer1"
+     transform="translate(0,-932.36218)">
+    <path
+       style="fill:none;stroke:#000000;stroke-width:3.29999995;stroke-linecap:butt;stroke-linejoin:miter;stroke-miterlimit:6.5;stroke-opacity:1;stroke-dasharray:none;marker-end:url(#Arrow2Lend)"
+       d="M 8.4275546,1036.5038 C 83.58314,1034.0277 96.349322,1007.92 111.03204,941.56937"
+       id="path2993"
+       inkscape:connector-curvature="0"
+       sodipodi:nodetypes="cc" />
+  </g>
+</svg>