Bidirectional parsing of canonical s-expressions

This is kept around for compatibility reasons. Before we serialised Pp.t tags as (Tag pp) but now we serialise them as Tag (pair tag pp).

A compound user error defineds an alternative format for error messages that retains more structure. This can be used to display the errors in richer form by RPC clients.

Describe what files should be promoted. The second argument of These is a function that is called on files that cannot be promoted.

Notification sent from server to client