Squashed 'teach.md/' content from commit 4060091

git-subtree-dir: teach.md
git-subtree-split: 406009130a8f0ffbdae41c4825d3b84eb996f8b5
Merge commit 'c1054b55bef6d077efc092cb03f0fe0bbb5e13d9' as 'teach.md'
Stop ignoring teach.md/src