First, a bit of personal history. Two weeks before October 19, 1987, I warned publicly that “in two weeks, the stock market will lose 20 percent of its value.” Then, on October 19, 1987, the S&P 500 had the biggest one-day fall in its history — dropping 20 percent. I was immediately deluged with letters and phone calls (no emails then) from people who wanted to sign up for my “investment letter.” I told them, with some regret, that I didn’t have an investment letter. What I didn’t tell them was that I had been making the same prediction — that in two weeks the stock market would lose 20 percen…