GF4 is written in Python. A look at any of the GF4 source files will show a number of comment lines scattered throughout the file. These are not normal comment lines; here are a few examples:
#@+leo-ver=5-thin
#@+node:tom.20211207165051.2: * @file gf4.pyw
#@+others
#@+node:tom.20211207165051.3: ** Imports
#@+node:tom.20211207165051.4: ** class PlotManager(AbstractPlotManager)
All these lines start with the prefix #@. What in the world are these? These
special comment lines are metadata inserted by the Leo-Editor IDE (Leo for
short). Leo is much more than an IDE. As an IDE, Leo lets you overlay structure
onto a file or project beyond the usual breakdown into classes, methods, and
functions. This along with Leo's structure handling abilities (among other
capabilities) makes understanding and managing large and complex code bases much
easier than most other editors and IDEs. These metadata comment lines are called
sentinels.
The sentinel lines tell Leo, when it reads a file, how to create the overlay structure. Do not move or change them!
Since these lines are comments, they will not affect execution in any way. But they can be annoying when reading the code outside of Leo. It is possible for Leo to write these files without sentinels. Leo can load the files and re-apply the overlay structure. But the results will be less robust against significant restructuring, since Leo may not be able to match up old and new material as intended. This could lead to a loss of some of the overlay structure (not the code itself).
The creator of GF4 has elected to keep the sentinel lines in the files. This decision could be revisited at some point. In the meantime, files can be edited outside of Leo as long as the sentinels are not moved or changed.
Ideally developers would install Leo and use it for all work on GF4. But Leo has a significant learning curve, and if GF4 is to be the only use for Leo, the effort may not be worthwhile.
For those interested in taking a look at Leo - and Leo is a very fine tool for development and producing documentation - it is on Github.