Return a list of transitive edges.
    
    Example of transitivity within graphs: A -> B, B -> C, A ->  C
    in this case the transitive edge is: A -> C
    
    @attention: This function is only meaningful for directed acyclic graphs.
    
    @type graph: digraph
    @param graph: Digraph
    
    @rtype: List
    @return: List containing tuples with transitive edges (or an empty array if the digraph
        contains a cycle)

def transitive_edges(graph):
    """
    Return a list of transitive edges.
    
    Example of transitivity within graphs: A -> B, B -> C, A ->  C
    in this case the transitive edge is: A -> C
    
    @attention: This function is only meaningful for directed acyclic graphs.
    
    @type graph: digraph
    @param graph: Digraph
    
    @rtype: List
    @return: List containing tuples with transitive edges (or an empty array if the digraph
        contains a cycle) 
    """
    #if the graph contains a cycle we return an empty array
    if not len(find_cycle(graph)) == 0:
        return []
    
    tranz_edges = [] # create an empty array that will contain all the tuples
    
    #run trough all the nodes in the graph
    for start in topological_sorting(graph):
        #find all the successors on the path for the current node
        successors = [] 
        for a in traversal(graph,start,'pre'):
            successors.append(a)
        del successors[0] #we need all the nodes in it's path except the start node itself
        
        for next in successors:
            #look for an intersection between all the neighbors of the 
            #given node and all the neighbors from the given successor
            intersect_array = _intersection(graph.neighbors(next), graph.neighbors(start) )
            for a in intersect_array:
                if graph.has_edge((start, a)):
                    ##check for the detected edge and append it to the returned array   
                    tranz_edges.append( (start,a) )      
    return tranz_edges