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;
}