Anathema Character Engine

This is a collection of design notes and thoughts on the Anathema Character Engine.
The engine is not yet implemented, it is growing in the two Engine_ modules: Engine_Character is a generic engine for modelling characters in roleplaying games, while Engine_Exalted provides a sample implementation for Exalted.

Basic concepts

The engine models a character requires three different sets of entitites:

Qualities represent the state of the character. They consist of an identifier and a value. Common examples of Qualities are Attributes and Abilities, e.g. Strength = 4.
Qualities are not limited to numeric values. They can represent items from the character's arsenal as well as an Intimacy toward another character.

Commands change a character. Most common commands modify existing qualities, e.g. Set Valor to 3 or Finish character creation. Some commands can also add new qualities, e.g. Add a Background called Artifact.
Characters remember their history of commands, so players can modify them later on.

Rules govern the interaction of Commands and Qualities. They check the character's state after the command has happened and decide whether the command is legal or not. Examples of rules are Attributes may not rise above 5 and You must be Exalted for at least 100 years before raising your Essence to 6.