. "metaphor for that which could not exist" . . . "black swan problem" . "Black swan theory" . "Black Swans" . . "black swan" . . . "tail risk hedging" . "Black swan theory" . "black swans" . "Black Swan" . . . "tail-risk events" . . "Black Swan idea" . "black swan theory" . . "black swan event" . "unicorn" . .