Doubly linked lists do not work very well in functional languages.# when you have a list [1,2,3] and you prepend a value [9,1,2,3] the language relies on the fact that the list tail [1,2,3] is not chainged by this operation. This means the list can be "prepended-in-place". With a doubly linked list the [1] cell would be changes (the 'left' pointer is changed from null to point to the [9] cell)... The result of this is that with a doubly linked list the _whole_ list must be copied when prepending a single value. In other words in a functional language, a doubly linked list will make ':' as hard as '++' and not the other way round! Keean.