Interface that defines one method (see snapshot) for ensuring that we get an instance that does not allow modifying state of `this` instance. Instance returned may be `this` if (and only if) it is immutable through its API (or, for some limited circumstances, if usage is guaranteed not to modify it after this point -- such usage is discouraged however); or, if that cannot be guaranteed, a newly created copy with same configuration and state as `this`.

Interface is intended to be used for persisting state for serialization, or to support "re-build" of otherwise immutable objects like factories. Some of the helper objects factories use and rely on have mutable state which cannot be shared: instead, to support re-building, a state object may be created to contain copies (snapshots). Intent, therefore, is that caller does not need to know about immutability (or lack thereof) of an entity but can simply call snapshot.