Increases the <defs/> definition of the given node »pnode«. This defs section is used to define linearGradients, radialGradients, Styles and images.
Images are later used with <use/>.
Example 8.6. ::dumper::arrow using addToDefs
...
# Put the shape into the marker.
svgdom::addElement path \
[concat \
[list d $shape($type)] $properties] {} \
[svgdom::addToDefs marker \
[list id $ID refX $refx($type) orient "auto" markerUnits userSpaceOnUse markerWidth $b markerHeight \
[expr { 2*$c}] viewBox $view($type)] \
[svgdom::getNodeByID $window \
[svgdom::getRoot $_doc]]]