Description: Define the class of all undirected pseudographs. An (undirected)
pseudograph consists of a set v (of "vertices") and a function e
(representing indexed "edges") into subsets of v of cardinality one
or two, representing the two vertices incident to the edge, or the one
vertex if the edge is a loop. This is according to Chartrand, Gary and
Zhang, Ping (2012): "A First Course in Graph Theory.", Dover, ISBN
978-0-486-48368-9, section 1.4, p. 26: "In a pseudograph, not only are
parallel edges permitted but an edge is also permitted to join a vertex
to itself. Such an edge is called a loop." (in contrast to a
multigraph, see df-umgr ). (Contributed by Mario Carneiro, 11-Mar-2015)(Revised by AV, 24-Nov-2020)