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+(** relations as values a subsystem hands to the tree. *)
+
+type heading = Protocols.Schematics.attribute_description BatMap.String.t
+type rows = (Concepts.Tuple.t list, Concepts.Condition.condition) result
+
+(** a relation holding [tuples], which never changes. *)
+val fixed : heading -> Concepts.Tuple.t list -> Protocols.Handle.t
+
+(** a relation computed from a directory of relations. Read as it is, it
+ is computed against [current ()] each time; read through a snapshot,
+ it is computed against the snapshot instead (see [Protocols.Derived]),
+ so it is what it was, or would be, at that moment. *)
+val computed :
+ heading ->
+ current:(unit -> (Protocols.Handle.t, Concepts.Condition.condition) result) ->
+ (Protocols.Handle.t -> rows) ->
+ Protocols.Handle.t
+
+(** the heading and tuples of a relation, read whole. *)
+val read :
+ Protocols.Handle.t -> (heading * Concepts.Tuple.t list, Concepts.Condition.condition) result