A <a href="https://en.wikipedia.org/wiki/Convex_set" class="extiw" title="en:Convex set">Concave set</a>
<a href="//commons.wikimedia.org/wiki/User:Oleg_Alexandrov" title="User:Oleg Alexandrov">Oleg Alexandrov</a>
self-made, with <a href="https://en.wikipedia.org/wiki/Inkscape" class="extiw" title="en:Inkscape">Inkscape</a>. Replaced an older version of the picture made by me also.