I represent a collection of links, which are containers for other objects. Using the message sequence addFirst:/removeLast causes the receiver to behave as a stack; using addLast:/removeFirst causes the receiver to behave as a queue.

	adding
	add: addFirst: addLast:

	enumerating
	do:

	removing
	remove:ifAbsent: removeFirst removeLast

	testing
	isEmpty

	accessing	top
	first Answer the first link. Create an error notification if the receiver is empty.
	last Answer the last link. Create an error notification if the receiver is empty.

	adding	top
	add: Add aLink to the end of the receiver's list. Answer aLink.
	addFirst: Add aLink to the beginning of the receiver's list. Answer aLink.
	addLast: Add aLink to the end of the receiver's list. Answer aLink.

	enumerating	top
	do: Refer to the comment in Collection|do:.

	removing	top
	remove:ifAbsent: Remove aLink from the receiver. If it is not there, answer the result of evaluating aBlock.
	removeFirst Remove the first element and answer it. If the receiver is empty, create an error notification.
	removeLast Remove the receiver's last element and answer it. If the receiver is empty, create an error notification.

	testing	top
	isEmpty Answer whether the receiver contains any elements.