Depth-first search subfunction for traversals.
    
    @type  graph: graph, digraph
    @param graph: Graph.

    @type  visited: dictionary
    @param visited: List of nodes (visited nodes are marked non-zero).

    @type  node: node
    @param node: Node to be explored by DFS.

def _dfs(graph, visited, node, pre, post):
    """
    Depth-first search subfunction for traversals.
    
    @type  graph: graph, digraph
    @param graph: Graph.

    @type  visited: dictionary
    @param visited: List of nodes (visited nodes are marked non-zero).

    @type  node: node
    @param node: Node to be explored by DFS.
    """
    visited[node] = 1
    if (pre): yield node
    # Explore recursively the connected component
    for each in graph[node]:
        if (each not in visited):
            for other in _dfs(graph, visited, each, pre, post):
                yield other
    if (post): yield node