The only real threat to the US market (specially for lower segments like pinky land) comes from inside...
...from (former) transfer agents like Select America printing tons of counterfeit shares, MMs playing endless games (for instance often not showing the best customer bids / asks, as we can see nearly dayly in PNMS) and much more of this kind.
But some people here prefer to close both eyes concerning these kind of (bad) activities.
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