Clean pre-existing links in output directory.

def prepare_output(d):
    """Clean pre-existing links in output directory."""
    outDir = os.path.join(d, 'inorder')
    if not os.path.exists(outDir):
        os.mkdir(outDir)
    for f in os.listdir(outDir):
        f = os.path.join(outDir, f)
        if os.path.islink(f):
            os.remove(f)
    return outDir