diff -ur unison-2.40.128.old/fs.ml unison-2.40.128/fs.ml
--- unison-2.40.128.old/fs.ml	2010-04-15 18:29:31.000000000 +0100
+++ unison-2.40.128/fs.ml	2017-02-14 12:52:50.260156414 +0000
@@ -21,7 +21,7 @@
 type dir_handle = System.dir_handle
                 = { readdir : unit -> string; closedir : unit -> unit }
 
-let symlink l f = System.symlink l (Fspath.toString f)
+let symlink ?to_dir l f = System.symlink l (Fspath.toString f)
 
 let readlink f = System.readlink (Fspath.toString f)
 
diff -ur unison-2.40.128.old/system/system_intf.ml unison-2.40.128/system/system_intf.ml
--- unison-2.40.128.old/system/system_intf.ml	2010-04-15 18:29:31.000000000 +0100
+++ unison-2.40.128/system/system_intf.ml	2017-02-14 12:53:27.127978087 +0000
@@ -20,7 +20,7 @@
 type fspath
 type dir_handle = { readdir : unit -> string; closedir : unit -> unit }
 
-val symlink : string -> fspath -> unit
+val symlink : ?to_dir:bool -> string -> fspath -> unit
 val readlink : fspath -> string
 val chown : fspath -> int -> int -> unit
 val chmod : fspath -> int -> unit