def _first_unvisited(graph, visited):
for each in graph:<EOL><INDENT>if (each not in visited):<EOL><INDENT>return each<EOL><DEDENT><DEDENT>return None<EOL>
Return first unvisited node.

@type  graph: graph
@param graph: Graph.

@type  visited: list
@param visited: List of nodes.

@rtype:  node
@return: First unvisited node.