^^SVG Scalable Vector Graphics.

Specifically, W3C recommends (and most browsers require) that for an SVG document to be properly formed the <svg> tag must contain a namespace declaration:

the agent reading the document as XML, now knows what XML namespace to use to interpret it.

Add an additional namespace declarations:

<svg xmlns="http://www.w3.org/2000/svg"
 xmlns:xlink="http://www.w3.org/1999/xlink">

this allows to use xlink (as in <use xlink:href="OBJ" />)