coalesce notes #4

|0x1|0x2|0x3| |prev: NULL|prev: 0x1|prev: 0x2| |next: 0x2|next: 0x3|next: NULL| (insert 0x4 between 0x2 and 0x3) last = 0x2 last -> next = new last -> next -> prev = new new -> prev = last new -> next = last -> next new = 0x4 last = 0x3 _new -> prev = last -> prev_ _new -> next = last_ last -> prev = new last -> prev -> next = new |0x1|0x2|0x4|0x3| |prev: NULL|prev: 0x1|prev: 0x2|prev: 0x4| |next: 0x2|next: 0x4|next: 0x3|next: NULL|