private static FieldValue asString(List<FieldValue> outputs) {
        StringBuilder ret = new StringBuilder();
        for (FieldValue val : outputs) {
            if (val == null) {
                return null;
            }
            ret.append(val);
        }
        return new StringFieldValue(ret.toString());
    }
We're either concatenating strings, or collections.
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/expressions/CatExpression.java