("  Area:         ", shape:area())
  if oo.instanceof(shape, Sphere) then
    print("  Volume:        ", shape:volume())
  end
end

c = Circle{ radius = 20.25 }
s = Sphere{ radius = 20.25 }

show(c)
show(s)
</pre>

<h2><a name="multiple">Multiple Model</a></h2>

<p>The third is the <code>multiple</code> model that enables the definition of classes with multiple inheritance. The <code>class</code> function of the <code>multiple</code> model takes a sequence of optional arguments that defines the set of super