Revision control
++ <svg width="800" height="400" viewBox="0 0 200 100" xmlns="http://www.w3.org/2000/svg">
++ <defs>
++ <symbol
++ id="mySymbol"
++ width="100"
++ height="100"
++ opacity="0.5"
++ >
++ <circle cx="25" cy="25" r="25" fill="red" />
++ <circle cx="50" cy="50" r="25" fill="red" />
++ </symbol>
++
++ <marker
++ id="myMarker"
++ markerWidth="100"
++ markerHeight="100"
++ opacity="0.5"
++ >
++ <circle cx="25" cy="25" r="25" fill="green" />
++ <circle cx="50" cy="50" r="25" fill="green" />
++ </marker>
++ </defs>
++
++ <use href="#mySymbol" x="0" y="0"/>
++
++ <polyline
++ points="100,0 100,100"
++ fill="none"
++ stroke="black"
++ marker-start="url(#myMarker)"
++ />
++ </svg>