private DataType getFirstStructOrMapRecursive() {
        DataType dataType = getDataType();
        while (dataType instanceof CollectionDataType) { // Currently no nesting of collections
            dataType = ((CollectionDataType)dataType).getNestedType();
        }
        return (dataType instanceof StructDataType || dataType instanceof MapDataType) ? dataType : null;
    }
Returns true if (there is strong evidence that) the value stored by this field is bit-packed.
config-model/src/main/java/com/yahoo/schema/document/SDField.java