Suggested by Rajiv, the general idea is this:

1. create a file objects
   a = File(Path("/dir/f.txt")
   b = File("http://hostname/f2.txt")

standard file paths assume site to be "local", for any other URL type, site does not matter

rc = ReplicaCatalog()
rc.add(a,b)

1. this would be the same as doing
   rc.add_replica("local", "f.txt", "/dir/f.txt")
   rc.add_replica("anysite", "f2.txt", "http://hostname/f2.txt")

1. but then convey the same amount of information with much less text