Class DS_TRAVERSABLE |

    indexing

>     description:

>         "Data structures that can be traversed. Traversable containers %
>         %are equipped with an internal cursor and external cursors which %
>         %can be used for simultaneous traversals."

>     library:    "Gobo Eiffel Structure Library"
>     author:     "Eric Bezault <ericb@gobosoft.com>"
>     copyright:  "Copyright (c) 1999-2001, Eric Bezault and others"
>     license:    "Eiffel Forum License v2 (see forum.txt)"

    deferred class interface

>     DS_TRAVERSABLE [G]

    inherit

>     DS_CONTAINER [G]

    feature {NONE} -- Initialization

>     make_default
>             -- Create an empty container.
>             -- (From DS_CONTAINER.)
>         deferred
>         ensure
>             empty: is_empty

    feature -- Access

>     item_for_iteration: G
>             -- Item at internal cursor position
>         require
>             not_off: not off

>     new_cursor: DS_CURSOR [G]
>             -- New external cursor for traversal
>         deferred
>         ensure
>             cursor_not_void: Result /= Void
>             valid_cursor: valid_cursor (Result)

    feature -- Measurement

>     count: INTEGER
>             -- Number of items in container
>             -- (From DS_CONTAINER.)
>         deferred

    feature -- Status report

>     is_empty: BOOLEAN
>             -- Is container empty?
>             -- (From DS_CONTAINER.)

>     off: BOOLEAN
>             -- Is there no item at internal cursor position?

>     same_position (a_cursor: like new_cursor): BOOLEAN
>             -- Is internal cursor at same position as a_cursor?
>         require
>             a_cursor_not_void: a_cursor /= Void

>     valid_cursor (a_cursor: DS_CURSOR [G]): BOOLEAN
>             -- Is a_cursor a valid cursor?
>         require
>             a_cursor_not_void: a_cursor /= Void

    feature -- Comparison

>     is_equal (other: like Current): BOOLEAN
>             -- Is current container equal to other?
>             -- (From GENERAL.)
>         require
>             other_not_void: other /= Void
>         deferred
>         ensure
>             consistent: standard_is_equal (other) implies Result
>             same_type: Result implies same_type (other)
>             symmetric: Result implies other.is_equal (Current)
>             same_count: Result implies count = other.count

    feature -- Duplication

>     copy (other: like Current)
>             -- Copy other to current container.
>             -- (From GENERAL.)
>         require
>             other_not_void: other /= Void
>             type_identity: same_type (other)
>         deferred
>         ensure
>             is_equal: is_equal (other)

    feature -- Cursor movement

>     go_to (a_cursor: like new_cursor)
>             -- Move internal cursor to a_cursor's position.
>         require
>             cursor_not_void: a_cursor /= Void
>             valid_cursor: valid_cursor (a_cursor)
>         ensure
>             same_position: same_position (a_cursor)

    feature -- Removal

>     wipe_out
>             -- Remove all items from container.
>             -- (From DS_CONTAINER.)
>         deferred
>         ensure
>             wiped_out: is_empty

    invariant

>     positive_count: count >= 0
>     empty_definition: is_empty = (count = 0)
>         -- (From DS_CONTAINER.)

>     empty_constraint: is_empty implies off

    end -- class DS_TRAVERSABLE