indexing
	description: "Trees implemented using a two way linked list representation"
	names: two_way_tree, tree, two_way_list
	representation: recursive, linked
	access: cursor, membership
	contents: generic

class interface
	TWO_WAY_TREE [G]

create 

	make (v: like item)
			-- Create single node with item v.

feature -- Initialization

	make (v: like item)
			-- Create single node with item v.
	
feature -- Access

	child_cursor: CURSOR
			-- Current cursor position
			-- (from LINKED_LIST)

	first: like child_item
			-- Item at first position
			-- (from LINKED_LIST)
		require -- from CHAIN
			not_empty: not is_leaf

	first_child: like parent
			-- Leftmost child

	has (v: G): BOOLEAN
			-- Does subtree include v?
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from TREE)
		ensure -- from CONTAINER
			not_found_in_empty: Result implies not is_leaf

	i_th (i: INTEGER): like child_item
			-- Item at i-th position
			-- Was declared in CHAIN as synonym of i_th and @.
			-- (from CHAIN)
		require -- from TABLE
			valid_key: valid_index (k)

	child_index: INTEGER
			-- Index of current position
			-- (from LINKED_LIST)
		ensure -- from TREE
			valid_index: Result >= 0 and Result <= arity + 1

	index_of (v: like child_item; i: INTEGER): INTEGER
			-- Index of i-th occurrence of item identical to v.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- 0 if none.
			-- (from CHAIN)
		require -- from LINEAR
			positive_occurrences: i > 0
		ensure -- from LINEAR
			non_negative_result: Result >= 0

	is_sibling (other: like parent): BOOLEAN
			-- Are current node and other siblings?
			-- (from TREE)
		ensure -- from TREE
			not_root: Result implies not is_root;
			other_not_root: Result implies not other.is_root;
			same_parent: Result = not is_root and other.parent = parent

	child_item: G
			-- Current item
			-- (from LINKED_LIST)
		require -- from ACTIVE
			readable: child_readable
		require -- from TRAVERSABLE
			not_off: not child_off
		require -- from TREE
			readable: child_readable

	item: G
			-- Content of cell.
			-- (from CELL)

	last: like child_item
			-- Item at last position
			-- (from LINKED_LIST)
		require -- from CHAIN
			not_empty: not is_leaf

	last_child: like parent

	left_sibling: like Current
			-- Left neighbor
			-- (from BI_LINKABLE)

	sequential_occurrences (v: G): INTEGER
			-- Number of times v appears.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from LINEAR)
		ensure -- from BAG
			non_negative_occurrences: Result >= 0

	occurrences (v: like child_item): INTEGER
			-- Number of times v appears.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from CHAIN)
		ensure -- from BAG
			non_negative_occurrences: Result >= 0

	occurrences (v: like child_item): INTEGER
			-- Number of times v appears.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from CHAIN)
		ensure -- from BAG
			non_negative_occurrences: Result >= 0

	parent: TWO_WAY_TREE [G]
			-- Parent node

	right_sibling: like Current
			-- Right neighbor
			-- (from LINKABLE)

	sublist: like Current
			-- Result produced by last split
			-- (from TWO_WAY_LIST)

	infix "@" (i: INTEGER): like child_item
			-- Item at i-th position
			-- Was declared in CHAIN as synonym of i_th and @.
			-- (from CHAIN)
		require -- from TABLE
			valid_key: valid_index (k)
	
feature -- Measurement

	count: INTEGER
			-- Number of items
			-- (from TREE)

	arity: INTEGER
			-- Number of items
			-- (from LINKED_LIST)
	
feature -- Status report

	child_after: BOOLEAN
			-- Is there no valid cursor position to the right of cursor?
			-- (from LINKED_LIST)

	child_before: BOOLEAN
			-- Is there no valid cursor position to the left of cursor?
			-- (from LINKED_LIST)

	changeable_comparison_criterion: BOOLEAN
			-- May object_comparison be changed?
			-- (Answer: yes by default.)
			-- (from CONTAINER)

	child_isfirst: BOOLEAN
			-- Is cursor under first child?
			-- (from TREE)
		ensure -- from TREE
			not_is_leaf: Result implies not is_leaf
		ensure -- from CHAIN
			valid_position: Result implies not is_leaf
		ensure -- from TREE
			not_is_leaf: Result implies not is_leaf

	child_islast: BOOLEAN
			-- Is cursor under last child?
			-- (from TREE)
		ensure -- from TREE
			not_is_leaf: Result implies not is_leaf
		ensure -- from CHAIN
			valid_position: Result implies not is_leaf
		ensure -- from TREE
			not_is_leaf: Result implies not is_leaf

	child_readable: BOOLEAN
			-- Is there a current child_item to be read?
			-- (from TREE)

	child_writable: BOOLEAN
			-- Is there a current child_item that may be modified?
			-- (from TREE)

	empty: BOOLEAN
			-- Is structure empty of items?
			-- (from TREE)

	exhausted: BOOLEAN
			-- Has structure been completely explored?
			-- (from LINEAR)
		ensure -- from LINEAR
			exhausted_when_off: child_off implies Result

	Extendible: BOOLEAN is true
			-- May new items be added?
			-- (from DYNAMIC_TREE)

	Child_extendible: BOOLEAN is true
			-- May new items be added? (Answer: yes.)
			-- (from DYNAMIC_CHAIN)

	is_leaf: BOOLEAN
			-- Are there no children?
			-- (from TREE)

	is_root: BOOLEAN
			-- Is there no parent?
			-- (from TREE)

	object_comparison: BOOLEAN
			-- Must search operations use equal rather than =
			-- for comparing references? (Default: no, use =.)
			-- (from CONTAINER)

	child_off: BOOLEAN
			-- Is there no current item?
			-- (from LINKED_LIST)

	prunable: BOOLEAN
			-- May items be removed? (Answer: yes.)
			-- (from DYNAMIC_CHAIN)

	Readable: BOOLEAN is true
			-- (from TREE)

	readable_child: BOOLEAN
			-- Is there a current child to be read?
			-- (from TREE)

	valid_cursor (p: CURSOR): BOOLEAN
			-- Can the cursor be moved to position p?
			-- (from LINKED_LIST)

	valid_cursor_index (i: INTEGER): BOOLEAN
			-- Is i correctly bounded for cursor movement?
			-- (from TREE)
		ensure -- from TREE
			valid_cursor_index_definition: Result = (i >= 0) and (i <= arity + 1)
		ensure -- from CHAIN
			valid_cursor_index_definition: Result = ((i >= 0) and (i <= arity + 1))
		ensure -- from TREE
			valid_cursor_index_definition: Result = (i >= 0) and (i <= arity + 1)

	valid_index (i: INTEGER): BOOLEAN
			-- Is i within allowable bounds?
			-- (from CHAIN)
		ensure then -- from CHAIN
			valid_index_definition: Result = ((i >= 1) and (i <= arity))

	Writable: BOOLEAN is true
			-- Is there a current item that may be modified?
			-- (from TREE)

	writable_child: BOOLEAN
			-- Is there a current child that may be modified?
			-- (from TREE)
	
feature -- Status setting

	compare_objects
			-- Ensure that future search operations will use equal
			-- rather than = for comparing references.
			-- (from CONTAINER)
		require -- from CONTAINER
			changeable_comparison_criterion
		ensure -- from CONTAINER
			object_comparison

	compare_references
			-- Ensure that future search operations will use =
			-- rather than equal for comparing references.
			-- (from CONTAINER)
		require -- from CONTAINER
			changeable_comparison_criterion
		ensure -- from CONTAINER
			reference_comparison: not object_comparison
	
feature -- Cursor movement

	child_back
			-- Move cursor to previous position, if any.
			-- (from TWO_WAY_LIST)
		require -- from BILINEAR
			not_before: not child_before
		ensure then -- from BILINEAR
			moved_back: child_index = old child_index - 1

	child_finish
			-- Move cursor to last position.
			-- (Go before if empty)
			-- (from TWO_WAY_LIST)
		ensure then -- from CHAIN
			at_last: not is_leaf implies child_islast
		ensure then -- from LINKED_LIST
			empty_convention: is_leaf implies child_before
		ensure then -- from TWO_WAY_LIST
			not_after: not child_after
		ensure then -- from TREE
			is_last_child: not is_leaf implies child_islast
		ensure then -- from TWO_WAY_LIST
			not_after: not child_after

	child_forth
			-- Move cursor to next position, if any.
			-- (from TWO_WAY_LIST)
		require -- LIST
			precursor: True
		require -- from LINEAR
			not_after: not child_after
		ensure then -- from LIST
			moved_forth: child_index = old child_index + 1

	child_go_i_th (i: INTEGER)
			-- Move cursor to i-th position.
			-- (from LINKED_LIST)
		require -- from CHAIN
			valid_cursor_index: valid_cursor_index (i)
		require else -- from TREE
			valid_cursor_index: valid_cursor_index (i)
		ensure -- from CHAIN
			position_expected: child_index = i
		ensure then -- from TREE
			position: child_index = i;
			is_before: (i = 0) implies child_before;
			is_after: (i = arity + 1) implies child_after

	child_go_to (p: CURSOR)
			-- Move cursor to position p.
			-- (from LINKED_LIST)
		require -- from CURSOR_STRUCTURE
			cursor_position_valid: valid_cursor (p)

	move (i: INTEGER)
			-- Move cursor i positions. The cursor
			-- may end up off if the offset is to big.
			-- (from TWO_WAY_LIST)
		ensure -- from CHAIN
			too_far_right: (old child_index + i > arity) implies exhausted;
			too_far_left: (old child_index + i < 1) implies exhausted;
			expected_index: (not exhausted) implies (child_index = old child_index + i)
		ensure then -- from LINKED_LIST
			moved_if_inbounds: ((old child_index + i) >= 0 and (old child_index + i) <= (arity + 1)) implies child_index = (old child_index + i);
			before_set: (old child_index + i) <= 0 implies child_before;
			after_set: (old child_index + i) >= (arity + 1) implies child_after

	search_child (v: like child_item)
			-- Move to first position (at or after current
			-- position) where item and v are equal.
			-- If structure does not include v ensure that
			-- exhausted will be true.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from BILINEAR)
		ensure -- from LINEAR
			object_found: (not exhausted and object_comparison) implies equal (v, child_item);
			item_found: (not exhausted and not object_comparison) implies v = child_item

	child_start
			-- Move cursor to first position.
			-- (from LINKED_LIST)
		ensure then -- from CHAIN
			at_first: not is_leaf implies child_isfirst
		ensure then -- from LINKED_LIST
			empty_convention: is_leaf implies child_after
		ensure then -- from TREE
			is_first_child: not is_leaf implies child_isfirst
		ensure then -- from LINKED_LIST
			empty_convention: is_leaf implies child_after
	
feature -- Element change

	append (s: SEQUENCE [G])
			-- Append a copy of s.
			-- (from SEQUENCE)
		require -- from SEQUENCE
			argument_not_void: s /= void
		ensure -- from SEQUENCE
			new_count: arity >= old arity

	extend (v: like item)
			-- Add v as new child.
			-- (from DYNAMIC_TREE)

	child_extend (v: like child_item)
			-- Add v to end.
			-- Do not move cursor.
			-- (from TWO_WAY_LIST)
		require -- from COLLECTION
			extendible: child_extendible
		ensure -- from COLLECTION
			item_inserted: has (v)
		ensure then -- from BAG
			one_more_occurrence: occurrences (v) = old (occurrences (v)) + 1

	fill (other: TREE [G])
			-- Fill with as many items of other as possible.
			-- The representations of other and current node
			-- need not be the same.
			-- (from TREE)

	force (v: like child_item)
			-- Add v to end.
			-- (from SEQUENCE)
		require -- from SEQUENCE
			extendible: child_extendible
		ensure then -- from SEQUENCE
			new_count: arity = old arity + 1;
			item_inserted: has (v)

	twl_merge_left (other: like Current)
			-- Merge other into current structure before cursor
			-- position. Do not move cursor. Empty other.
			-- (from TWO_WAY_LIST)
		require -- from DYNAMIC_CHAIN
			extendible: child_extendible;
			not_off: not child_before;
			other_exists: other /= void
		ensure -- from DYNAMIC_CHAIN
			new_count: arity = old arity + old other.arity;
			new_index: child_index = old child_index + old other.arity;
			other_is_empty: other.is_leaf

	twl_merge_right (other: like Current)
			-- Merge other into current structure after cursor
			-- position. Do not move cursor. Empty other.
			-- (from TWO_WAY_LIST)
		require -- from DYNAMIC_CHAIN
			extendible: child_extendible;
			not_off: not child_after;
			other_exists: other /= void
		ensure -- from DYNAMIC_CHAIN
			new_count: arity = old arity + old other.arity;
			same_index: child_index = old child_index;
			other_is_empty: other.is_leaf

	merge_tree_after (other: like first_child)
			-- Merge children of other into current structure
			-- after cursor position. Do not move cursor.
			-- Make other a leaf.
		require -- from DYNAMIC_TREE
			not_child_off: not child_off;
			other_exists: (other /= void)
		ensure -- from DYNAMIC_TREE
			other_is_leaf: other.is_leaf

	merge_tree_before (other: like first_child)
			-- Merge children of other into current structure
			-- after cursor position. Do not move cursor.
			-- Make other a leaf.
		require -- from DYNAMIC_TREE
			not_child_off: not child_off;
			other_exists: (other /= void)
		ensure -- from DYNAMIC_TREE
			other_is_leaf: other.is_leaf

	prune (n: like first_child)
		require -- from TREE
			is_child: n.parent = Current
		ensure -- from TREE
			n_is_root: n.is_root

	put (v: like item)
			-- Make v the cell's item.
			-- Was declared in CELL as synonym of put and replace.
			-- (from CELL)
		require -- CELL
			precursor: True
		require -- from TREE
			is_writable: writable
		ensure -- from CELL
			item_inserted: item = v
		ensure -- from TREE
			item_inserted: item = v
		ensure -- from CELL
			item_inserted: item = v

	child_put (v: like child_item)
			-- Replace current item by v.
			-- (Synonym for replace)
			-- (from CHAIN)
		require -- from COLLECTION
			extendible: child_extendible
		require -- from TREE
			child_writable: child_writable
		ensure -- from COLLECTION
			item_inserted: has (v)
		ensure then -- from CHAIN
			same_count: arity = old arity
		ensure -- from TREE
			item_inserted: child_item = v
		ensure then -- from CHAIN
			same_count: arity = old arity

	put_child (n: like parent)
			-- Add n to the list of children.
			-- Do not move child cursor.
		require else -- from DYNAMIC_TREE
			non_void_argument: n /= void

	put_child_left (n: like parent)
			-- Add n to the left of cursor position.
			-- Do not move cursor.
		require -- from DYNAMIC_TREE
			not_child_before: not child_before;
			non_void_argument: n /= void

	put_child_right (n: like parent)
			-- Add n to the right of cursor position.
			-- Do not move cursor.
		require -- from DYNAMIC_TREE
			not_child_after: not child_after;
			non_void_argument: n /= void

	put_front (v: like child_item)
			-- Add v to beginning.
			-- Do not move cursor.
			-- (from TWO_WAY_LIST)
		ensure -- from DYNAMIC_CHAIN
			new_count: arity = old arity + 1;
			item_inserted: first = v

	put_i_th (v: like child_item; i: INTEGER)
			-- Put v at i-th position.
			-- (from CHAIN)
		require -- from TABLE
			valid_key: valid_index (k)
		ensure then -- from INDEXABLE
			insertion_done: i_th (k) = v

	child_put_left (v: like child_item)
			-- Add v to the left of cursor position.
			-- Do not move cursor.
			-- (from TWO_WAY_LIST)
		require -- from DYNAMIC_CHAIN
			extendible: child_extendible;
			not_before: not child_before
		require -- from DYNAMIC_TREE
			not_child_before: not child_before
		ensure -- from DYNAMIC_CHAIN
			new_count: arity = old arity + 1;
			new_index: child_index = old child_index + 1

	child_put_right (v: like child_item)
			-- Add v to the right of cursor position.
			-- Do not move cursor.
			-- (from TWO_WAY_LIST)
		require -- from DYNAMIC_CHAIN
			extendible: child_extendible;
			not_after: not child_after
		require -- from DYNAMIC_TREE
			not_child_after: not child_after
		ensure -- from DYNAMIC_CHAIN
			new_count: arity = old arity + 1;
			same_index: child_index = old child_index
		ensure then -- from LINKED_LIST
item_inserted_before: old child_before implies child.item = v

	replace (v: like item)
			-- Make v the cell's item.
			-- Was declared in CELL as synonym of put and replace.
			-- (from CELL)
		require -- CELL
			precursor: True
		require -- from TREE
			is_writable: writable
		ensure -- from CELL
			item_inserted: item = v
		ensure -- from TREE
			item_inserted: item = v
		ensure -- from CELL
			item_inserted: item = v

	child_replace (v: like child_item)
			-- Replace current item by v.
			-- (from LINKED_LIST)
		require -- from ACTIVE
			writable: child_writable
		require -- from TREE
			child_writable: child_writable
		ensure -- from ACTIVE
			item_replaced: child_item = v
		ensure -- from TREE
			item_inserted: child_item = v

	replace_child (n: like parent)
			-- Replace current child by n.
		require -- from TREE
			writable_child: writable_child;
			was_root: n.is_root
		ensure -- from TREE
			child_replaced: child = n

	sprout
			-- Make current node a root.
			-- (from TREE)
	
feature -- Removal

	twl_prune (v: like child_item)
			-- Remove first occurrence of v, if any,
			-- after cursor position.
			-- If found, move cursor to right neighbor;
			-- if not, make structure exhausted.
			-- (from DYNAMIC_CHAIN)
		require -- from COLLECTION
			prunable: prunable

	prune_all (v: like child_item)
			-- Remove all occurrences of v.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- Leave structure exhausted.
			-- (from DYNAMIC_CHAIN)
		require -- DYNAMIC_CHAIN
			precursor: True
		require -- from COLLECTION
			prunable
		ensure then -- from DYNAMIC_CHAIN
			is_exhausted: exhausted
		ensure -- from COLLECTION
			no_more_occurrences: not has (v)
		ensure then -- from DYNAMIC_CHAIN
			is_exhausted: exhausted

	remove_child
			-- Remove current item.
			-- Move cursor to right neighbor
			-- (or after if no right neighbor).
			-- (from TWO_WAY_LIST)
		require -- from ACTIVE
			prunable: prunable;
			writable: child_writable
		require -- from DYNAMIC_TREE
			child_not_off: not child_off
		ensure then -- from DYNAMIC_LIST
			after_when_empty: is_leaf implies child_after
		ensure -- from DYNAMIC_TREE
			new_arity: arity = old arity - 1;
			new_child_index: child_index = old child_index

	remove_left_child
			-- Remove item to the left of cursor position.
			-- Do not move cursor.
			-- (from TWO_WAY_LIST)
		require -- from DYNAMIC_CHAIN
			left_exists: child_index > 1
		require else -- from DYNAMIC_LIST
			not_before: not child_before
		require -- from DYNAMIC_TREE
			is_not_first: not child_isfirst
		ensure -- from DYNAMIC_CHAIN
			new_count: arity = old arity - 1;
			new_index: child_index = old child_index - 1
		ensure -- from DYNAMIC_TREE
			new_arity: arity = old arity - 1;
			new_child_index: child_index = old child_index - 1

	remove_right_child
			-- Remove item to the right of cursor position.
			-- Do not move cursor.
			-- (from TWO_WAY_LIST)
		require -- from DYNAMIC_CHAIN
			right_exists: child_index < arity
		require -- from DYNAMIC_TREE
			is_not_last: not child_islast
		ensure -- from DYNAMIC_CHAIN
			new_count: arity = old arity - 1;
			same_index: child_index = old child_index
		ensure -- from DYNAMIC_TREE
			new_arity: arity = old arity - 1;
			new_child_index: child_index = old child_index

	remove_sublist
			-- (from TWO_WAY_LIST)

	split (n: INTEGER)
			-- Remove from current list
			-- min (n, count - index - 1) items
			-- starting at cursor position.
			-- Move cursor right one position.
			-- Make extracted sublist accessible
			-- through attribute sublist.
			-- (from TWO_WAY_LIST)
		require -- from TWO_WAY_LIST
			not_off: not child_off;
			valid_sublist: n >= 0

	wipe_out
			-- Remove all items.
			-- (from TWO_WAY_LIST)
		require -- from COLLECTION
			prunable
		ensure -- from COLLECTION
			wiped_out: is_leaf
		ensure then -- from DYNAMIC_LIST
			is_before: child_before
	
feature -- Transformation

	swap (i: INTEGER)
			-- Exchange item at i-th position with item
			-- at cursor position.
			-- (from CHAIN)
		require -- from CHAIN
			not_off: not child_off;
			valid_index: valid_index (i)
		ensure -- from CHAIN
			swapped_to_item: child_item = old i_th (i);
			swapped_from_item: i_th (i) = old child_item
	
feature -- Conversion

	binary_representation: BINARY_TREE [G]
			-- Convert to binary tree representation:
			-- first child becomes left child,
			-- right sibling becomes right child.
			-- (from TREE)
		ensure -- from TREE
			result_is_root: Result.is_root;
			result_has_no_right_child: not Result.has_right

	fill_from_binary (b: BINARY_TREE [G])
			-- Fill from a binary tree representation.
			-- Left child becomes first child.
			-- Right child becomes right sibling.
			-- Any right child of b is ignored.
			-- (from DYNAMIC_TREE)

	linear_representation: LINEAR [G]
			-- Representation as a linear structure
			-- (from TREE)
	
feature -- Duplication

	duplicate (n: INTEGER): like Current
			-- Copy of sub-tree beginning at cursor position and
			-- having min (n, arity - child_index + 1)
			-- children
			-- (from DYNAMIC_TREE)
		require -- from TREE
			not_child_off: not child_off;
			valid_sublist: n >= 0
	
feature -- Implementation

	child: like first_child
			-- Element at cursor position
			-- (from LINKED_LIST)
	
invariant

		-- from GENERAL
	reflexive_equality: standard_is_equal (Current);
	reflexive_conformance: conforms_to (Current);
	off_constraint: (child = void) implies child_off;
		-- from DYNAMIC_TREE
	extendible_definition: extendible;
	child_after_definition: child_after = (child_index = arity + 1);
		-- from TREE
	leaf_definition: is_leaf = (arity = 0);
	child_off_definition: child_off = child_before or child_after;
	child_before_definition: child_before = (child_index = 0);
	child_isfirst_definition: child_isfirst = (not is_leaf and child_index = 1);
	child_islast_definition: child_islast = (not is_leaf and child_index = arity);
	child_after_definition: child_after = (child_index >= arity + 1);
	child_consistency: child_readable implies child.parent = Current;
		-- from BI_LINKABLE
	right_symmetry: (right_sibling /= void) implies (right_sibling.left_sibling = Current);
	left_symmetry: (left_sibling /= void) implies (left_sibling.right_sibling = Current);
		-- from LINKED_LIST
	prunable: prunable;
	empty_constraint: is_leaf implies ((first_child = void) and (child = void));
	not_void_unless_empty: (child = void) implies is_leaf;
	before_constraint: child_before implies (child = first_child);
	after_constraint: child_after implies (child = last_child);
		-- from LIST
	before_definition: child_before = (child_index = 0);
	after_definition: child_after = (child_index = arity + 1);
		-- from CHAIN
	non_negative_index: child_index >= 0;
	index_small_enough: child_index <= arity + 1;
	off_definition: child_off = ((child_index = 0) or (child_index = arity + 1));
	isfirst_definition: child_isfirst = ((not is_leaf) and (child_index = 1));
	islast_definition: child_islast = ((not is_leaf) and (child_index = arity));
	item_corresponds_to_index: (not child_off) implies (child_item = i_th (child_index));
		-- from ACTIVE
	writable_constraint: child_writable implies child_readable;
	empty_constraint: is_leaf implies (not child_readable) and (not child_writable);
		-- from BILINEAR
	not_both: not (child_after and child_before);
	empty_property: is_leaf implies (child_after or child_before);
	before_constraint: child_before implies child_off;
		-- from LINEAR
	after_constraint: child_after implies child_off;
		-- from TRAVERSABLE
	empty_constraint: is_leaf implies child_off;
		-- from FINITE
	empty_definition: is_leaf = (arity = 0);
	non_negative_count: arity >= 0;
		-- from DYNAMIC_CHAIN
	extendible: child_extendible;

end -- class TWO_WAY_TREE