import java.util.Collection;

/**
 * Interface specifying a graph
 */
public interface Graph {
  
  /** 
   * Return the collection of vertices of this graph
   * @return the vertices as a collection (which is anything iterable)
   */
  public Collection vertices();
 
  /**
   * Return the collection of edges of this graph
   * @return the edges as a collection (which is anything iterable)
   */
  public Collection edges();
  
  /**
   * Return a collection of edges incident to (i.e. emanating from) 
   * a given vertex v.
   *      i.e., the set of all edges v->w 
   * @param v one of the vertices in the graph
   * @return an iterable collection of edges incident to v in the graph
   */
  public Collection incidentEdges(Vertex v);
  
  /**
   * Test whether two vertices are adjacent in the graph
   * @param v one vertex
   * @param w another vertex
   * @return true if there is an edge from v to w in the graph, false otherwise
   */
  public boolean isAdjacent(Vertex v, Vertex w);
}