A vertex pair (A, B) is said to be reciprocal if there are
edges between them in both directions. The reciprocity of a directed
graph is the proportion of all possible (A, B) pairs which are
reciprocal, provided there is at least one edge between A and B. The
reciprocity of an empty graph is undefined (results in an error
code). Undirected graphs always have a reciprocity of 1.0 unless they
are empty.