A transparent box with polygons. It shows that a mathematical set can be seen as the content of an imaginary box.
<ul><li><a href="//commons.wikimedia.org/wiki/File:Package-x-generic.svg" title="File:Package-x-generic.svg">File:Package-x-generic.svg</a>: <a rel="nofollow" class="external text" href="http://tango.freedesktop.org/The_People">The people from the Tango! project</a></li>
<li><a href="//commons.wikimedia.org/wiki/File:Example_of_a_set.svg" title="File:Example of a set.svg">File:Example of a set.svg</a>: <a rel="nofollow" class="external text" href="http://kulla.me">Stephan Kulla</a> (<a href="//commons.wikimedia.org/wiki/User:Stephan_Kulla" title="User:Stephan Kulla">User:Stephan Kulla</a>)
<ul><li><a href="//commons.wikimedia.org/wiki/File:PolygonsSet.svg" title="File:PolygonsSet.svg">File:PolygonsSet.svg</a>: <a href="//commons.wikimedia.org/wiki/User:Kismalac" title="User:Kismalac">kismalac</a></li></ul></li>
<li>derivative work: <a rel="nofollow" class="external text" href="http://kulla.me">Stephan Kulla</a> (<a href="//commons.wikimedia.org/wiki/User:Stephan_Kulla" title="User:Stephan Kulla">User:Stephan Kulla</a>)</li></ul>
2015-05-03
<p>This work based on:
</p>
<ul><li><a href="//commons.wikimedia.org/wiki/File:Package-x-generic.svg" title="File:Package-x-generic.svg">File:Package-x-generic.svg</a></li>
<li><a href="//commons.wikimedia.org/wiki/File:Example_of_a_set.svg" title="File:Example of a set.svg">File:Example of a set.svg</a>
<ul><li>derivative work of <a href="//commons.wikimedia.org/wiki/File:PolygonsSet.svg" title="File:PolygonsSet.svg">File:PolygonsSet.svg</a></li></ul></li></ul>