Common Lisp ships with a set of powerful built in data structures including
 the venerable list, full featured arrays, and hash-tables. CL-containers
 enhances and builds on these structures in two ways:
 .
 By adding containers that are not available in native Lisp (for example:
 binary search trees, red-black trees, sparse arrays and so on).
 .
 By providing a standard interface so that they are simpler to use and so
 that changing design decisions becomes significantly easier.