def checkbox_property(question):
    """
    Convert a checkbox question into JSON Schema.
    """
    return {question['id']: {
        "type": "array",
        "uniqueItems": True,
        "minItems": 0 if question.get('optional') else 1,
        "maxItems": question.get('number_of_items', len(question['options'])),
        "items": {
            "enum": [
                option.get('value', option['label'])
                for option in question['options']
            ]
        }
    }}