[Haskell-cafe] Relative files paths

Suppose that I'm in foo/bar/box1/top and I want to access a file in foo/bar/box2/side. How do I construct the shortest possible relative path from one to the other? The filepath package doesn't seem to provide a function to do this. (The correct answer is obviously "../../box2/side" in this instance, but how do I compute that in general?)