This page will contain all important things about Sophie 2.0 persisting things.
 * immutables.
 * property persisting.
 * format registry.
 * supported formats.
 * ways of persisting for every format.
 * diagrams.
 * extension points for formats and way of creating extensions.
 * XML structures.
 * etc.
 * every format added and every extension to some format should be described here.