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book/src/interop-environment.md

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+# Interop Environment
+
+All that is required for inter-op is a built and tested [development
+environment](./setup.md).
+
+## Repositories
+
+You will only require the [sigp/lighthouse](http://github.com/sigp/lighthouse)
+library.
+
+To allow for faster build/test iterations we will use the
+[`interop`](https://github.com/sigp/lighthouse/tree/interop) branch of
+[sigp/lighthouse](https://github.com/sigp/lighthouse/tree/interop) for
+September 2019 interop.  **Please use ensure you `git checkout interop` after
+cloning the repo.**
+
+## File System
+
+When lighthouse boots, it will create the following
+directories:
+
+- `~/.lighthouse`: database and configuration for the beacon node.
+- `~/.lighthouse-validator`: database and configuration for the validator
+	client.
+
+After building the binaries with `cargo build --release --all`, there will be a
+`target/release` directory in the root of the Lighthouse repository. This is
+where the `beacon_node` and `validator_client` binaries are located.
+
+You do not need to create any of these directories manually.