docs: 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