hex_fmt is copyright 2018, POA Networks, Ltd.

Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
http://www.apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
http://opensource.org/licenses/MIT>, at your option. All files in the project
carrying such notice may not be copied, modified, or distributed except
according to those terms.