- Provides analogous functionality to a doubly-linked list
  implementation.
- Completely symmetric set of operations.
- Operations on the ADT do not lead to results that are
  outside the domain of the ADT (closed interface).
- Alternative to container/list.

R=rsc
DELTA=489  (489 added, 0 deleted, 0 changed)
OCL=32284
CL=32323