sub write_out {
    my $graph = shift;
    my $file = 'graph.dump';
    open my $out', '>', $file or die "Cannot open $file for output: $!\n";
    print $out $graph->_dump;
    close $out or die "Could not close output $file: $!\n";
}

##</code><code>##

sub read_in {
    my $file = 'graph.dump';
    open my $in', '<', $file or die "Cannot open $file for input: $!\n";
    my $Graph;
    local $/ = undef;
    # eval {  <$in> }; # sets $Graph
    eval <$in>; # sets $Graph
    close $in;
    return $Graph;
}