before delete:
   +------+ +------+
   V      | V      |
+--+--+  ++-+--+  ++-+--+
|  | -+->|| | -+->|| |  |
+--+--+  +--+--+  +--+--+
prev     current  next

after delete:
   +---------------+
   V               |
+--+--+  +--+--+  ++-+--+
|  | |+  |  | -+->|| |  |
+--+-++  +--+--+  +--+--+
     |               ^
     +---------------|

prev     current  next