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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _optional_envs():
 """ Parse environment variables for optional values, raising a `BrowserConfig` error if they are insufficiently specified. Returns a `dict` of environment variables. """

    envs = {
        key: os.environ.get(key)
        for key in OPTIONAL_ENV_VARS
        if key in os.environ
    }

    # If we're using Jenkins, check that we have all the required info
    if 'JOB_NAME' in envs and 'BUILD_NUMBER' not in envs:
        raise BrowserConfigError("Missing BUILD_NUMBER environment var")

    if 'BUILD_NUMBER' in envs and 'JOB_NAME' not in envs:
        raise BrowserConfigError("Missing JOB_NAME environment var")

    return envs