sub print_tree {
    my ( $tree, $parent, $chain ) = ( @_, $EMPTY, $EMPTY );
    ref $tree eq "HASH" or return;
    for ( sort keys %$tree ) {
        print /^\s*\z/ ? "<blank>" : "$parent;". "$chain"."$_", "\n";
        print_tree( $tree->{$_},  $_, "$chain"."$_"."_");
    }
}