use Test::More tests => 4;
use Graph;
my $g = Graph->new;
ok( $g->add_vertex("a") );
ok( $g->add_vertex("b") );
is( $g->add_vertex("c"), $g );
eval '$g->add_vertex("c", "d")';
like($@,
qr/Graph::add_vertex: use add_vertices for more than one vertex/);