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Citation networks form directed acyclic graphs. Visualisation of the reference relationships within the network requires fast determination of ancestor-descendant relationships among the nodes within the digraph. Polytrees (singly connected digraphs) constitute an important class of acyclic digraphs and can be viewed as generalised directed trees where multiple parent nodes are permitted. In this paper, we propose a new ancestral numbering scheme for polytrees that allows fast determination of ancestor-descendant relationships among the nodes without traversal. The scheme is used to index and visualise reference relationships within citation networks.