elpa-seq software package provides sequence manipulation functions for Emacs Lisp, 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-seq
elpa-seq is installed in your system.
Make ensure the elpa-seq package were installed using the commands given below,
$ sudo dpkg-query -l | grep elpa-seq *
You will get with elpa-seq package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Lisp |
| Package | elpa-seq |
| Version | 2.16-1 |
| Description | sequence manipulation functions for Emacs Lisp |
| Sub Section | Lisp |