[Unison-hackers] Does copyprog option do not work for local files?

[netawater]

I try use unison to synchronize my files with copyprog option, I
have set copythreshold = 0, but It seems unison does not call my 
copy program. 

I have studied the copy.ml, and found these expressions in file function

match rootFrom, rootTo with
    (Common.Local, fspathFrom), (Common.Local, realFspathTo) ->
      localFile
        fspathFrom pathFrom fspathTo pathTo realPathTo
        update desc (Osx.ressLength ress) (Some id);
      Lwt.return ()

It means if it found there are local files, it will call localFile function
so that copyprog is ignored.

I tried to skip these expressions, but it cause it call targetExistsOnRoot
which can not work in local file environment.

I know little about Ocaml, I hope someone can help me, Thanks in advance.