@staticmethod<EOL><INDENT>def truncateGraph(graph, root_nodes):<DEDENT>
subgraph = Graph()<EOL>for node in root_nodes:<EOL><INDENT>subgraph = GraphUtils.joinGraphs(subgraph, GraphUtils.getReacheableSubgraph(graph, node))<EOL><DEDENT>return subgraph<EOL>
Create a set of all nodes containg the root_nodes and
           all nodes reacheable from them