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source/_posts/MissingSemester/Lecture6.md

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 Commits in Git are immutable. This doesn’t mean that mistakes can’t be corrected, however; it’s just that “edits” to the commit history are actually creating entirely new commits, and references (see below) are updated to point to the new ones.
 
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 ## Data model, as pseudocode
 
 It may be instructive to see Git’s data model written down in pseudocode:
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 It’s a clean, simple model of history.
 
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 ## Objects and content-addressing
 
 An “object” is a blob, tree, or commit:
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 git is wonderful
 ```
 
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 ## References
 
 Now, all snapshots can be identified by their SHA-1 hashes. That’s inconvenient, because humans aren’t good at remembering strings of 40 hexadecimal characters.
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 One detail is that we often want a notion of “where we currently are” in the history, so that when we take a new snapshot, we know what it is relative to (how we set the `parents` field of the commit). In Git, that “where we currently are” is a special reference called “HEAD”.
 
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 ## Repositories
 
 Finally, we can define what (roughly) is a Git _repository_: it is the data `objects` and `references`.
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 Whenever you’re typing in any command, think about what manipulation the command is making to the underlying graph data structure. Conversely, if you’re trying to make a particular kind of change to the commit DAG, e.g. “discard uncommitted changes and make the ‘master’ ref point to commit `5d83f9e`”, there’s probably a command to do it (e.g. in this case, `git checkout master; git reset --hard 5d83f9e`).
 
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 ## Staging area
 
 This is another concept that’s orthogonal to the data model, but it’s a part of the interface to create commits.
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 Git accommodates such scenarios by allowing you to specify which modifications should be included in the next snapshot through a mechanism called the “staging area”.
 
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 ## Exercises
 
 1. Clone the [repository for the class website](https://github.com/missing-semester/missing-semester).