Description: Define the class of all undirected simple pseudographs (which could have
loops). An undirected simple pseudograph is a special undirected
pseudograph (see uspgrupgr ) or a special undirected simple hypergraph
(see uspgrushgr ), consisting of a set v (of "vertices") and an
injective (one-to-one) 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.
In contrast to a pseudograph, there is at most one edge between two
vertices resp. at most one loop for a vertex. (Contributed by Alexander
van der Vekens, 10-Aug-2017)(Revised by AV, 13-Oct-2020)