def _realpath(fs, path, seen=pset()):
    """
    .. warning::

        The ``os.path`` module's realpath does not error or warn about
        loops, but we do, following the behavior of GNU ``realpath(1)``!
    """

    real = Path.root()
    for segment in path.segments:
        current = real / segment
        seen = seen.add(current)
        while True:
            try:
                current = fs.readlink(current)
            except (exceptions.FileNotFound, exceptions.NotASymlink):
                break
            else:
                current = current.relative_to(real)
                if current in seen:
                    raise exceptions.SymbolicLoop(path)
                current = fs.realpath(current, seen=seen)
        real = current
    return real