Dummy Store Type

1st chunk of `src/libstore/dummy-store.md`

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**Store URL format**: `dummy://`

This store type represents a store that contains no store paths and
cannot be written to. It's useful when you want to use the Nix
evaluator when no actual Nix store exists, e.g.

```console
# nix eval --store dummy:// --expr '1 + 2'
```

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Title: Dummy Store Type

Summary

The `dummy://` store type represents a read-only, empty store. It's used for evaluation purposes when a real Nix store isn't needed, like in simple calculations.