set:		{ 1 2 3 4 6 8 9 }
erase(3):	{ 1 2 4 6 8 9 }
multiset:	{ 1 1 1 1 2 3 3 4 4 6 8 9 }
count(1) = 4
find(4) = 7
find(5) = false
erase(3):	{ 1 1 1 1 2 4 4 6 8 9 }