more: Other extreme ideas mooted by the financial authorities include a tax on physical currency or introducing one to operate alongside the yen. All three ideas are based on a theory concerning interest rates and the concept that a nominal rate of zero — as Japan has now lived with for much of the past decade — may be too high. In Japan’s case, the theory would suggest that nominal rates of -4 percent might be closer to what is required to rescue the economy from another..

Hmmm - that would wipe an entire underground economy here.