Merge branch 'section_7_1' into section_8_1
typo fix
Merge branch 'section_6_4' into section_6_5
Merge branch 'section_6_5' into section_6_6
Merge branch 'section_6_6' into section_6_7
Merge branch 'section_6_7' into section_6_7a
Merge branch 'section_6_7a' into section_6_8
Merge branch 'section_6_8' into section_7_1
Merge branch 'section_7_1' into section_8_1
Merge branch 'section_8_1' into section_8_4
Merge branch 'section_8_4' into section_9_1

Conflicts:
	.gitignore
typo
test fix
Merge branch 'section_8_1' into section_8_4
Merge branch 'section_8_4' into section_9_1
Merge branch 'section_9_1' into section_9_2
Merge branch 'section_9_3' into section_9_2