public static BooleanSetUnmarshaller instance() {
        return INSTANCE;
    }
A special unmarshaller for Set&lt;Boolean>, which the V1 schema stores as
an NS using 0/1 for true/false. In the V2 schema these fall through to
the {@code ObjectSetToListMarshaller} which stores them as an L or BOOLs.
aws-java-sdk-dynamodb/src/main/java/com/amazonaws/services/dynamodbv2/datamodeling/unmarshallers/BooleanSetUnmarshaller.java