Adding lecture13
Merge branch 'master' of https://github.com/AllenDowney/SoftwareSystems
Adding solution
Adding Sync.py
Quick fix
Adding lecture 14
Log2 scale
Merge branch 'master' of https://github.com/AllenDowney/SoftwareSystems
Bug fix in Sync.py
Adding lecture15
Adding hw04 soln
Adding code for quiz 4
Merge branch 'master' of https://github.com/AllenDowney/SoftwareSystems
Adding list2_soln.c
Adding lecture18
Merge branch 'master' of https://github.com/AllenDowney/SoftwareSystems
Adding lecture19
Adding mutex example
Merge branch 'master' of https://github.com/AllenDowney/SoftwareSystems
Fixing matrix_soln.c