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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 _clean_fields(allowed_fields: dict, fields: FieldsParam) -> Iterable[str]: """Clean lookup fields and check for errors."""

    if fields == ALL:
        fields = allowed_fields.keys()
    else:
        fields = tuple(fields)
        unknown_fields = set(fields) - allowed_fields.keys()
        if unknown_fields:
            raise ValueError('Unknown fields: {}'.format(unknown_fields))
    return fields