Description: The vertex degree of a one-edge graph, case 4: an edge from a vertex to
itself contributes two to the vertex's degree. I. e. in a graph (simple
pseudograph) with one edge which is a loop (see uspgr1v1eop ), the
vertex connected with itself by the loop has degree 2. (Contributed by Mario Carneiro, 12-Mar-2015)(Revised by Alexander van der Vekens, 22-Dec-2017)(Revised by AV, 17-Dec-2020)(Proof shortened by AV, 21-Feb-2021)