Format describes the range of valid documents. It consists of the following interfaces:
|Validator||provides yes/no answers on the validity of proposed structures|
|Synthesizer||generates valid structures based on proposed structures|
|Extender||generated more valid structures based on a currently valid structure|
|Metadata||provides additional information on nodes, such as configuration for other modules.|
The default Format expects the metadata to be able to resolve key 'content-model' to the ContentModel for a given node, or to null if the node need not be validated.
Optional Validator instances to restrict the Format beyond its content models.