== Causal order ==
Some user-sent message specifies messages specify a "parent" sequence number which is the last message the user received before sending it. Note:
* If Alice sends messages (A,B) in a row, A will not be B's parent unless Alice waits till A is received back from the server.
* The parent of a message is different from the "previous" message in the server's ordering, e.g. in a "simultaneous send" case two messages will have the same parent.