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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 standard_licenses_only(build_context, target) -> str: """A policy function for allowing specifying only known licenses. Return error message (string) if policy for `target` is violated, otherwise return `None`. To apply in project, include this function in the ilst returned by the `get_policies` function implemented in the project `YSettings` file. See example in tests/errors. """

    for license_name in target.props.license:
        if license_name not in KNOWN_LICENSES:
            # TODO: include suggestion for similar known license
            return 'Unknown license: {}'.format(license_name)
    return None