/**
 * @defgroup extension Extension Points
 * @brief    Available extension points for the uSCXML intepreter.
 *
 * @{
 * 	@defgroup ioproc I/O Processors
 * 	@brief    Custom In-/Ouput processors.

I/O processors will allow to send and receive events from and to a running
SCXML instance.

 * 	@defgroup invoker Invokers
 * 	@brief    Types of components for the invoke element to summon.

Invokers will allow to specify types of external components to instantiate and
communicate with via events.

 * 	@defgroup datamodel Data-Models
 * 	@brief    Additional data-models for embedded scriptings.

Data-models provide a syntax and semantic for an embedded scripting language.
Within the SCXML standard there are normative descriptions of ECMAScript and an
XPath description that was removed before recommendation status.

 * 	@defgroup element Executable Content
 * 	@brief    Custom XML elements for executable content.

An extension point to register custom elements as executable content.

 * @}

 * @defgroup interpreter Interpreter
 * @brief    The components constituting the core of the interpreter.
 * @{

 * 	@defgroup microstep Microstep
 * 	@brief    Implementations of the micro-step(t) algorithm.

 * 	@defgroup execcontent Content Executor
 * 	@brief    Instances to process executable content as part of a micro-step.

 * 	@defgroup eventqueue Event Queue
 * 	@brief    Implementation of proper and delayed event queues.

 * @}

 * @defgroup callback Callbacks
 * @brief    Callback interfaces.

 * @defgroup facade Facades
 * @brief    User-facing facades for the implementations.

 * @defgroup abstract Interfaces
 * @brief    Base classes as interfaces for implementations.

 */