Breaks any cycles which occur in the input graph by reversing the internal direction of certain cyclic edges.
Assigns nodes to discrete ranks or levels. In a top-to-bottom drawing, ranks determine Y coordinates. Edges that span more than one rank are broken into chains of “virtual” nodes and unit-length edges.
Orders nodes within ranks to avoid crossings.
Sets X coordinates of nodes to keep edges short, and the final step routes edge splines.
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