title: Obtain the vertices from a list of edges
body: <p>My list contains a number of facts as its elements.I want to search if a number is a member of the list or not.</p>
  
  <p>Here I defined the edge facts in my program.Now I want to check if the node 1 is a member of the list.</p>
  
  <pre><code>?-my([edge(1,2),edge(2,3),edge(1,4)],1,L).
  false.
  </code></pre>