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Extensions
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Extensions are integral part of SDC, allowing developers to enhance the data model provided with additional information. protoSDC supports extensions using the [Any](https://protobuf.dev/programming-guides/proto3/#any) well-known type.

Specifying Extensions
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`protoSDC Converter` supports generating both standalone extensions as well as extensions that reference elements from the BICEPS data model.

Generating the `BICEPS` data model using `proto-converter` produces a file called an `Episode`, which contains information about the generated model. This file can be used in subsequent executions of `proto-converter` to allow newly generated output to reference the already generated data model.

Take this example:

.. literalinclude:: ../example_schemas/example_extension.xsd
        :language: xml
        :linenos:

It imports a schema for which we might already have a generated data model and we do not want to generate it again. If so, we can configure the `proto-converter` to associate all references to `example.xsd` by changing our configuration to load the episode for example.xsd.

.. literalinclude:: ../config.toml.example.episode
        :language: toml
        :linenos:
        :lines: -13

This tells the converter to enhance the data model with data from the `episode_out.json` file, which contains the references to the existing `example.xsd` data model.

Extension best practices
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`proto-converter` only supports a subset of XML schema. For compatibility purposes, follow these rules:

- no `xsd:any`
- no `xsd:anyAttribute`
- no nested extensions
- ...