Bull markets climb a wall of worry, they say. That’s the old rule of thumb that leads many investors to buy stocks when a few green shoots appear under the black clouds. The fact that stocks historically turn up several months ahead of time also provides reassurance.

The stock market rally is quite extended. Will the reversal be shallow or protracted? There seems to be a lot of disagreement over this point, and more generally, whether this is a bear-market rally or the start of a new bull market.