Historically, the `Sparkle` strategy's `Item` struct has only
included basic values from the appcast that are commonly useful.
Over time we've selectively added/surfaced more values as we've
encountered outliers that require use of different values in a
`strategy` block.
We now need to use `minimumSystemValue`, so this expands the `Item`
struct to include any appcast value that we could conceivably want
to use in the future. This will hopefully save us from having to make
more modifications to the struct (and related tests) before we can
use a previously-unused value in a `strategy` block.
The main class you should look at is the {Formula} class (and classes linked from there). That's the class that's used to create Homebrew formulae (i.e. package descriptions). Assume anything else you stumble upon is private.
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