ass.
	The iteration variables hold only the internal object that represent each sub-class of <code>class</code>.
	Each sub-class is iterared only once.
	This method is used only by other class models that extend this one, <i>e.g.</i> the <code>scoped</code> model.</dd>
</dl>

<h3>Example</h3>

<pre>
CachedObject = oo.class()

function CachedObject:__index(field)
  local value = oo.classof(self)[field]
  if value then rawset(self, field, value) end
  return value
end

CachedSphere = oo.class({}, CachedObject