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def depth_first_search(v)
  raise ArgumentError, "No such vertex" if v < 0 or
vertices <= v
  is_visited = []
  visit = lambda do |v|
    each_edge(v) do |v,w|
      next if is_visited[w]
      yield v,w
      is_visited[w] = true
      visit.call(w)
end end
  yield -1,v
  is_visited[v] = true
  visit.call(v)
end