indexing
	description: "Collections of items, where each item may occur zero or more times, and the number of occurrences is meaningful."
	names: bag, access
	access: membership
	contents: generic

deferred class interface
	BAG [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
	
feature -- Measurement

	occurrences (v: G): INTEGER
			-- Number of times v appears in structure
			-- (Reference or object equality,
			-- based on object_comparison.)
		ensure
			non_negative_occurrences: Result >= 0
	
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)

	extendible: BOOLEAN
			-- May new items be added?
			-- (from COLLECTION)

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

	prunable: BOOLEAN
			-- May items be removed?
			-- (from COLLECTION)
	
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 -- Element change

	extend (v: G)
			-- Add a new occurrence of v.
		require -- from COLLECTION
			extendible: extendible
		ensure -- from COLLECTION
			item_inserted: has (v)
		ensure then
			one_more_occurrence: occurrences (v) = old (occurrences (v)) + 1

	fill (other: CONTAINER [G])
			-- Fill with as many items of other as possible.
			-- The representations of other and current structure
			-- need not be the same.
			-- (from COLLECTION)
		require -- from COLLECTION
			other_not_void: other /= void;
			extendible

	put (v: G)
			-- Ensure that structure includes v.
			-- Was declared in COLLECTION as synonym of put and extend.
			-- (from COLLECTION)
		require -- from COLLECTION
			extendible: extendible
		ensure -- from COLLECTION
			item_inserted: has (v)
	
feature -- Removal

	prune (v: G)
			-- Remove one occurrence of v if any.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from COLLECTION)
		require -- from COLLECTION
			prunable: prunable

	prune_all (v: G)
			-- Remove all occurrences of v.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from COLLECTION)
		require -- from COLLECTION
			prunable
		ensure -- from COLLECTION
			no_more_occurrences: not has (v)

	wipe_out
			-- Remove all items.
			-- (from COLLECTION)
		require -- from COLLECTION
			prunable
		ensure -- from COLLECTION
			wiped_out: empty
	
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);

end -- class BAG