* When serializing, the top level container, and all of its children, are required to be *strictly* [invariant][wiki_invariant] during enumeration. This property is used to make certain optimizations, such as if a particular object has already been serialized, the result of the previous serialized `UTF8` string can be reused (i.e., the `UTF8` string of the previous serialization can simply be copied instead of performing all the serialization work again). While this is probably only of interest to those who are doing extraordinarily unusual things with the run-time or custom classes inheriting from the classes that JSONKit will serialize (i.e, a custom object whose value mutates each time the it receives a message requesting its value for serialization), it also covers the case where any of the objects to be serialized are mutated during enumeration (i.e., mutated by another thread). The number of times JSONKit will request an objects value is non-deterministic, from a minimum of once up to the number of times it appears in the serialized JSON– therefore an object MUST NOT depend on receiving a message requesting its value each time it appears in the serialized output. The behavior is `undefined` if these requirements are violated.