Common abstract definition of a sequenceable collection of objects. Such a collection is traversable
using a simple INTEGER index from lower to upper using item. All COLLECTIONs are resizable
thanks to add_last / remove_last, add_first / remove_first as well as add / remove .
This abstraction provides feature to view a COLLECTION as a stack (as an example by using add_last,
last, and remove_last). One can also use a COLLECTION as a queue (as an example, by using
add_last, first and remove_first). See also class QUEUE and STACK.
The basic = is used
for comparison of items and indices are not considered (for
example this routine may yeld True with Current indexed in
range [1..2] and other indexed in range [2..3]).