list

早期freebsd实现

Using lists

	Lists are a sequence of values which are doubly linked so that
	elements can be removed or inserted anywhere within the list.
	The function 'list' creates a list with possible initial elements.
	For example,

		x = list(4, 6, 7);

	creates a list in the variable x of three elements, in the order
	4, 6, and 7.

	The 'push' and 'pop' functions insert or remove an element from
	the beginning of the list.  The 'append' and 'remove' functions
	insert or remove an element from the end of the list.  The 'insert'
	and 'delete' functions insert or delete an element from the middle
	(or ends) of a list.  The functions which insert elements return
	the null value, but the functions which remove an element return
	the element as their value.  The 'size' function returns the number
	of elements in the list.

	Note that these functions manipulate the actual list argument,
	instead of returning a new list.  Thus in the example:

		push(x, 9);

	x becomes a list of four elements, in the order 9, 4, 6, and 7.
	Lists can be copied by assigning them to another variable.

	An arbitrary element of a linked list can be accessed by using the
	double-bracket operator.  The beginning of the list has index 0.
	Thus in the new list x above, the expression x[[0]] returns the
	value of the first element of the list, which is 9.  Note that this
	indexing does not remove elements from the list.

	Since lists are doubly linked in memory, random access to arbitrary
	elements can be slow if the list is large.  However, for each list
	a pointer is kept to the latest indexed element, thus relatively
	sequential accesses to the elements in a list will not be slow.

	Lists can be searched for particular values by using the 'search'
	and 'rsearch' functions.  They return the element number of the
	found value (zero based), or null if the value does not exist in
	the list.