A <a href="https://en.wikipedia.org/wiki/Degeneracy_(graph_theory)" class="extiw" title="en:Degeneracy (graph theory)">2-degenerate graph</a> (all vertices have at most two neighbors to their left) and its 2-core (the subgraph remaining after repeatedly removing vertices of degree less than two)
<a href="//commons.wikimedia.org/wiki/User:David_Eppstein" class="mw-redirect" title="User:David Eppstein">David Eppstein</a>
<span class="int-own-work" lang="en">Own work</span>