elpa-async software package provides simple library for asynchronous processing in Emacs, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install elpa-async
elpa-async is installed in your system.
Make ensure the elpa-async package were installed using the commands given below,
$ sudo dpkg-query -l | grep elpa-async *
You will get with elpa-async package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Lisp |
| Package | elpa-async |
| Version | 1.9-2 |
| Description | simple library for asynchronous processing in Emacs |
| Sub Section | Lisp |