bsip32: add when to check for black swan event

bsip-0032.md

@@ -51,6 +51,29 @@ Matching between a limit order and a call order is done in
 `check_call_orders(...)` function of `database` class, price of limit order
 is always used. It need to be changed to use MSSP when the call order is maker.
 
+Currently a black swan event will occur when the call order with least
+collateral ratio is going to be matched below 100% collateral ratio price
+(name it CRP). Because the call order will be matched with incoming limit order
+at limit order's price (taker price),
+which can be higher or lower than CRP, so even if MSSP is
+below CRP, an incoming taker limit order may or may not trigger a black swan.
+So it makes sense to check if a black swan event will occur every time when a
+limit order is created.
+
+After the behavior changed to always match at maker price, when MSSP is below
+CRP, an incoming taker limit order will always trigger a black swan event.
+So it makes sense to trigger the black swan event when MSSP is below CRP rather
+than waiting for an incoming limit order. That means it's no longer needed to
+check for black swan event when a limit order is created. Since checking for
+black swan event is somehow expensive, we'll gain a side benefit on performance
+with the change.
+
+When triggering a black swan event when MSSP is below CRP, sometimes the short
+position with least collateral ratio may still have more than 100% collateral
+ratio. In this case, the global settlement price is CRP but not the actual
+collateral ratio of the short position with least collateral ratio.
+This is current behavior, it's fair, no need to change.
+
 # Discussion
 
 [to be added if any]