lly: never use one or two letter property names
  except for "x" and "y" on <a>Point</a>, <a>Rect</a>, <a>Spot</a>, and <a>LayoutVertex</a> instances
  and the <a>InputEvent.up</a> property.
</p>
<p>
  Do not modify the prototypes of any of the <b>GoJS</b> classes.
  If you modify the built-in classes, we cannot support you.
  The way to modify the behavior of the <b>GoJS</b> classes is via the techniques discussed at
  <a href="extensions.html">Extensions</a>.
  However most of the <b>GoJS</b> classes cann