INTERNAL API Optimization for add/remove followed by merge and merge should just fast forward to the new instance. It's like a cache between calls of the same thread, you can think of it as a thread local. The Replicator actor invokes the user's modify function, which returns a new ReplicatedData instance, with the ancestor field set (see for example the add method in ORSet). Then (in same thread) the Replication calls merge, which makes use of the ancestor field to perform quick merge (see for example merge method in ORSet). It's not thread safe if the modifying function and merge are called from different threads, i.e. if used outside the Replicator infrastructure, but the worst thing that can happen is that a full merge is performed instead of the fast forward merge.

public abstract class FastMerge<T> : IReplicatedData<T>, 
	IReplicatedData
where T : FastMerge<T>

[<AbstractClassAttribute>]
type FastMerge<'T when 'T : FastMerge<'T>> =  
    class
        interface IReplicatedData<'T>
        interface IReplicatedData
    end