`(graph-simple? (G <graph>))`methodPredicate for testing whether or not a graph G is simple. A graph is considered simple if it does not contain multiple edges and has no loops (i.e. edges that go from u->u). Given this definition, even if a graph has a type of <multidigraph> or <multigraph>, so long as no vertices actually _contain_ multiple edges,

`graph-simple?`can still evaluate to #t. If multiple edges or loops are present, then`graph-simple?`will evaluate to #f.