removed decoherence again, added it again and let Lazarus add the src to the search path and added decoherence manual to the lpr and got the warnings and hints. (messages too long to include here because of all the fgl.pp hints)
I've just tried this way... however, it didn't work either.
I removed decoherence.pas
I removed src from ProjectOptions
I added decohernce.pas
I added "src" folder to build modes
I added Decoherence to decoherence_desktop.lpr uses
I compiled the project and got warnings and hints (not all of them, but that was my fault, I think, because I did not purge the project, so it compiled only changed files)
I tried compiling/building/buildingClean/DeletingAllCompiledFiles again, and I got no warning/hints again.