code: def _dfs(graph, visited, count, node):
      
      visited[node] = count
      # Explore recursively the connected component
      for each in graph[node]:
          if (each not in visited):
              _dfs(graph, visited, count, each)
docs: Depth-first search subfunction adapted for accessibility algorithms.
      
      @type  graph: graph, digraph, hypergraph
      @param graph: Graph.
  
      @type  visited: dictionary
      @param visited: List of nodes (visited nodes are marked non-zero).
  
      @type  count: number
      @param count: Counter of connected components.
  
      @type  node: node
      @param node: Node to be explored by DFS.