camlp4-extra software package provides virtual package provided by camlp4, 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 camlp4-extra
camlp4-extra is installed in your system.
Make ensure the camlp4-extra package were installed using the commands given below,
$ sudo dpkg-query -l | grep camlp4-extra*
You will get with camlp4-extra package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Virtual packages |
| Package | camlp4-extra |
| Version | |
| Description | virtual package provided by camlp4 |
| Sub Section | Virtual |