Description: The union of two simple graphs (with the same vertex set): If
<. V , E >. and <. V , F >. are simple graphs, then
<. V , E u. F >. is a multigraph (not necessarily a simple graph!) -
the vertex set stays the same, but the edges from both graphs are kept,
possibly resulting in two edges between two vertices. (Contributed by AV, 29-Nov-2020)