sub di_graph_connected {
    my $g = shift; # $g is of type 'Graph'
    my @verts = $g->vertices;
    my @pairs = ();
    foreach my $front ( @verts ) {
        foreach my $back ( @verts ) {
            push @pairs, [ $front, $back ] if $g->has_edge( $front, $back );
        }
    }

    return @pairs;
}