fix exercise 1.1.2
ex 1.2.1 - fix
fix a mistake

we can't call a structured set in question "a category"
before we actually prove that it is a category.
ex 1.2.4 - done
1.2.5 - fix
1.2.6 - fix
ex 1.2.7 - fix
bump
1.3.2 - done
1.3.3 - done
separate completed exercises
1.3.4 - done
Merge branch 'intro-to-cat'