* {@link #addNode(Object) add} each missing endpoint to the graph.
   *
   * @return the value previously associated with the edge connecting {@code nodeU} to {@code
   *     nodeV}, or null if there was no such edge.
   * @throws IllegalArgumentException if the introduction of the edge would violate {@link
   *     #allowsSelfLoops()}
   * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed
   * @since 27.1
   */

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/*
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