elpa-flx-ido software package provides allows Emacs Ido to use the flx sorting algorithm, 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-flx-ido
elpa-flx-ido is installed in your system.
Make ensure the elpa-flx-ido package were installed using the commands given below,
$ sudo dpkg-query -l | grep elpa-flx-ido *
You will get with elpa-flx-ido package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Lisp |
| Package | elpa-flx-ido |
| Version | 0.6.1-1 |
| Description | allows Emacs Ido to use the flx sorting algorithm |
| Sub Section | Lisp |