font-ibm-plex has a nonfree dependency

> To build binary font files from .vfb sources you need FontLab Studio

> 5. A Python script called IBM Plex export FDK files.py is necessary

> to export the proper files from FontLab. To run this script you’ll

> need the RoboFab library. Also, you’ll need to have the Adobe Font

> Development Kit for OpenType (AFDKO) installed.

> In particular, a free system distribution should be self-hosting.

> This means that you must be able to develop and build the system

> with tools that the system provides you. As a result, a free system

> distribution cannot include free software that can only be built by

> using nonfree software.

> IBM releases Plex as "open source", but unfortunately the freedom

> status of the font is less than ideal, as it requires non-free

> software to be built from source (see README.md, Requirements section,

> and [0]). The problematic step is the hinting, which is done with

> FontLab Studio, which is proprietary software. There is currently no

> Free Software program that is able to work with the high-level hinting

> command language used by FontLab Studio.

> Hi, Guix developers:

>

> As far as I can tell, the IBM Plex family of fonts requires the

> nonfree program FontLab Studio to build.

> This is one of the reasons Debian[3] has moved the font to the "contrib"

> section of its repos.

>

>> IBM releases Plex as "open source", but unfortunately the freedom

>> status of the font is less than ideal, as it requires non-free

>> software to be built from source (see README.md, Requirements section,

>> and [0]). The problematic step is the hinting, which is done with

>> FontLab Studio, which is proprietary software. There is currently no

>> Free Software program that is able to work with the high-level hinting

>> command language used by FontLab Studio.

>

> The IBM Plex instances started their life as interpolated files in RoboFont which were checked and refined manually for rounding errors in outlines and metrics. Settings in the font info might have been tailored for specific instances as well. Afterwards they were converted to TrueType outlines, and together with the PostScript versions they were hinted in FontLab 5.

>

> If you are interested to work with the instances then I strongly recommend to take the FontLab files because these are of a much higher fidelity than plain interpolated instances.

>

> With a utility such as vfb2ufo these files can be converted to UFO files. Although you will probably loose most, if not all all, hinting data when doing so.

> I have my own Python implementation of a FontLab-style TTH compiler which can take an UFO (from vfb2ufo so it contains the FL TT hinting code) and compile the TT instructions. I would be open to release it if there are no objections from the FontLab guys.

> The alignment zone deltas are stored in the UFO generated by vfb2ufo, so they can be accessed there by any script working with the UFO.