Create a set of all nodes containg the root_nodes and
		   all nodes reacheable from them

def truncateGraph(graph, root_nodes):
		"""Create a set of all nodes containg the root_nodes and
		   all nodes reacheable from them
		"""
		subgraph = Graph()
		for node in root_nodes:
			subgraph = GraphUtils.joinGraphs(subgraph, GraphUtils.getReacheableSubgraph(graph, node))

		return subgraph