title: Checking the existence of an edge on an undirected graph using Lemon Graph Library
body: <p>Graph example:</p>
  
  <pre><code>ListDigraph G;
  
  ListGraph::Node A = G.addNode();
  ListGraph::Node B = G.addNode();
  ListGraph::Node C = G.addNode();
  ListGraph::Node D = G.addNode();
  
  ListGraph::Edge AB = G.addEdge(A,B);
  ListGraph::Edge AC = G.addEdge(A,C);
  ListGraph::Edge AD = G.addEdge(A,D);
  ListGraph::Edge BC = G.addEdge(B,C);
  ListGraph::Edge BD = G.addEdge(B,C);
  ListGraph::Edge CD = G.addEdge(C,D);
  </code></pre>
  
  <p>I need a method that takes 2 nodes (A and C) for example and returns the ID of the Edge that connects these 2 nodes (if it exists).</p>