finds the tail of a directed edge
graph or network
edge of G
set or list of edges of G
Important: The networks package has been deprecated.Use the superseding package GraphTheory instead.
If only G is passed then a table indexed by edges and recording the vertices at the tail of selected edges is returned. The table is part of the graph data structure and is automatically maintained by procedures such as addedge() and delete().
If the first argument is a directed edge e then the tail of e is returned. For undirected edges the result is NULL.
The routine maps automatically onto a set or list of vertices when the first argument is a set or list.
It is normally loaded via the command with(networks) but may also be referenced using the full name networks[tail](...).
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