No matter how much I try, I do not understand compatibility matrix of intent locking. Why were SIX and IX denied if x lock has been held? Suppose x lock holds for record r1 then taking IX just means that some record in the table held X lock? If so, I think it should not be denied. Why is IX denied if S lock has been held? Why is IX allowed if IS has been held? Why IS is allowed if IX has been held?