def METHOD_NAME():
    global _last
    s = format_stacks()
    snap = s
    if _last:
        snap += '\n'
        diff = list(difflib.unified_diff(
            a=_last.splitlines(),
            b=s.splitlines(),
            fromfile='then',
            tofile='now'
        ))
        if diff:
            snap += '\n'.join(diff) + '\n'
        else:
            snap += '(no change since last time)\n'
    _last = s
    return snap