etoolbox software package provides TeX Live: transitional dummy package, 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 etoolbox
etoolbox is installed in your system.
Make ensure the etoolbox package were installed using the commands given below,
$ sudo dpkg-query -l | grep etoolbox *
You will get with etoolbox package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | TeX |
| Package | etoolbox |
| Version | 2016.20161103-1 |
| Description | TeX Live: transitional dummy package |
| Sub Section | TeX |