I would like to move a particular composite shape to a <Symbol> definition, and then re-use it. This helps make the SVG code neater, and provides some global control over the actual structure of that shape. The shape is symmetrical about a given mid-point; however, the x/y coordinates of subsequent <Use> statements need to reference the top-left corner rather than its natural original, and this means that all usage of the shape must be aware of its total size. Is there a way to position a Symbol by some origin other than its top-left corner?

Contrived example, purely to explain this better. The following concentric-circle Symbol has a natural origin. However, the subsequent Use statement has to offset its x/y coordinates by half the Symbol size in order to position it correctly (at 20,20 in this example). Ideally, the usage of the symbol should not have to know this information.

```
<defs>
  <symbol id="ex">
    <circle  fill="green" cx="8" cy="8" r="8"/>
    <circle  fill="white" cx="8" cy="8" r="6"/>
    <circle  fill="green" cx="8" cy="8" r="4"/>
  </symbol>
</defs>

<use xlink:href="#ex" x="12" y="12">

```