Is it possible to ban payment for order flow without banning payment for order flow?

That is the question that has been raised by a series of changes to stock-market rules laid out by Securities and Exchange Commission chair Gary Gensler this week. None of them are a straightforward prohibition of payment for order flow — or PFOF — which is the hotly-debated practice by retail brokerages like Robinhood Markets or Charles Schwab of sending customer orders to market-making firms such as Citadel Securities or Virtu Financial and collecting payments from those firms in return.