Chapter 5: Integration
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Introduces higher-order forms.

Wedge products, which generate higher-order forms. Went through this pretty
fast, I think this section is where a bunch of tensor indexing tricks are
skipped, or would have been.

In this chapter, much of this content is only defined for euclidean R2 or R3,
need metrics for the general case.

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As a side note, having `define-coordinates` also define things like `d/dx` and
just `dx` feels like kind of a syntactic sugar trick.