how to find the cardinality of neighbors of a set of vertices in a graph by using sagemath?

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Recall the definition of a dominating set. If D is a dominating set of a graph G=(V,E), then for each vertex u\in V, either u is in D or it is a neighbor of a vertex in D. I assume that you are interested in the strict neighborhood of D, that is the set of vertices that are neighbors of a vertex in D but are not in D. Obviously, it's V-D and the cardinality of this set is obvious to deduce.