thought about introducing new variant selectIfExists - but this is not
consitent with the general logic ...
Maybe only if it is resolved to a Maybe<Node> ?!?!
exploring a bug in demo 03, synchronization, in which two sessions get
wrongly synchronized

this seemed to have been caused by concurrent modification (append while
messages from server are pending)

to fix this, a new method getSafe() has been added for entities.
Manipulating nodes in the callback assures that no synchronization
conflicts occur.