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Collects the "disappearing potential" associated with vertices that have
either no incoming edges, no outgoing edges, or both. Vertices that have
no incoming edges do not directly contribute to the hub scores of other
vertices; similarly, vertices that have no outgoing edges do not directly
contribute to the authority scores of other vertices. These values are
collected at each step and then distributed across all vertices as a part
of the normalization process. (This process is not required for, and does
not affect, the 'sum-of-squares'-style normalization.)