From Mr Tom Price.

Sir, Our masters, to popular acclaim, have banned the practice of borrowing an asset (shares) and using it to buy another (cash) when one expects that the cash will be worth more than be the borrowed shares. Their reason is that the market is irrational and there is a systemic risk. The cause of that systemic risk is the rapid decrease of prices that, a few years ago, were increasing rapidly (to popular acclaim) but with identical irrationality.