title: Can there be self loops for undirected graphs?
body: <p>I mean directed graphs can have a self-loop, so I don't see the reason why an undirected graph cannot have it (CLRS says it's forbidden without giving a valid reason). </p>
  
  <pre><code>Example: 
  
  G_directed = (V,E) is a directed graph
  
  Say this graph has the vertex set V = {1,2,3,4,5,6}
  
  With edges E = {(1,2),(2,2),(2,4),(2,5),(4,1),(4,5),(5,4),(6,3)}
  -----------------------------------------------------------------
  Say we now decide to turn G_directed into an undirected graph:
  
  G_undirected = (Vu,Eu) is an undirected graph
  
  Vu = {1,2,3,4,5,6}
  
  With edges E = {(1,2),(2,2),(2,4),(2,5),(4,1),(4,5),(6,3)}
  </code></pre>
  
  <p>In the example (2,2) is the <strong>self loop</strong>. I seriously don't see any problems this can have with graph traversals.</p>