indexing
	description: "Hierarchical structures in which each item has zero or one immediate predecessor, and zero or more successors."
	names: hierarchical, traversing
	access: cursor
	contents: generic

deferred class interface
	HIERARCHICAL [G]

feature -- Access

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

	item: G
			-- Item at current position
			-- (from TRAVERSABLE)
		require -- from TRAVERSABLE
			not_off: not off

	successor_count: INTEGER
			-- Number of successors of current element
		require
			not_off: not off
	
feature -- Status report

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

	empty: BOOLEAN
			-- Is there no element?
			-- (from CONTAINER)

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

	off: BOOLEAN
			-- Is there no current item?
			-- (from TRAVERSABLE)
	
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

	down (i: INTEGER)
			-- Move to i-th successor.
		require
			not_off: not off;
			argument_within_bounds: i >= 1 and i <= successor_count

	start
			-- Move to first position if any.
			-- (from TRAVERSABLE)

	up
			-- Move to predecessor.
		require
			not_off: not off
	
feature -- Conversion

	linear_representation: LINEAR [G]
			-- Representation as a linear structure
			-- (from CONTAINER)
	
invariant

		-- from GENERAL
	reflexive_equality: standard_is_equal (Current);
	reflexive_conformance: conforms_to (Current);
	non_negative_successor_count: successor_count >= 0;
		-- from TRAVERSABLE
	empty_constraint: empty implies off;

end -- class HIERARCHICAL