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For a number of pragmatic reasons it is convenient to single out the outermost region of the plane in a distinctive way and to mark it as the ''root node'' of the corresponding dual graph, indicated in the present text by means of a horizontal strike-through.

The outermost region of the plane-embedded graph is singled out for special consideration and the corresponding node of the dual graph is referred to as its ''root node''.  By way of graphical convention in the present text, the root node is indicated means by means of a horizontal strike-through.

Extracting the dual graph from its composite matrix, we get this picture:

Extracting the dual graph from its composite matrix, we get this picture: