def dump f=$stdout
    f.puts "=== start thread with #{@containers.length} trees ==="
    @containers.each { |c| c.dump_recursive f; f.puts }
    f.puts "=== end thread ==="
  end
def dump f=$stdout
    f.puts "=== start thread with #{@containers.length} trees ==="
    @containers.each { |c| c.dump_recursive f; f.puts }
    f.puts "=== end thread ==="
  end
unused