Adding hw04 soln
Adding code for quiz 4
Adding list2_soln.c
Adding lecture18
Merge branch 'master' of github.com:AllenDowney/SoftwareSystems
Adding lecture19
Adding mutex example
Fixing matrix_soln.c
Adding exam2
Fixing matrix2.c
Merge branch 'master' of github.com:AllenDowney/SoftwareSystems
Corrected errors in vector.c
Implemented is_magic_square