@Override
    public boolean isKeywordMultiField() {
        return !isKeyword() && fieldMap.containsKey(ES_MAPPING_TYPE_KEYWORD)
            && fieldMap.get(ES_MAPPING_TYPE_KEYWORD).isKeyword();
    }
This is current quite an implementation-specific mechanism, but we check: <p/>
 * If this top-level field is a keyword, in which case it's not a text-keyword multi-field
 * Otherwise, check to see if there's a "keyword" sub-entry with the type of keyword.
common/es/src/main/java/io/apiman/common/es/util/builder/index/EsIndexProperty.java