Merge branch 'master' of https://github.com/AllenDowney/SoftwareSystems
Adding lecture19
Adding mutex example
Merge branch 'master' of https://github.com/TheoT/SoftwareSystems
Merge branch 'master' of https://github.com/AllenDowney/SoftwareSystems
Merge branch 'master' of https://github.com/TheoT/SoftwareSystems
done with glib