> What you're asking is similar to the problem of finding the predecessor
> of an arbitrary node in a singly-linked-list.  You have no option but to
> scan the whole list to find its predecessor.  If you had a
> doubly-linked-list, predecessor lookups would work easily, but that's a
> different data structure, with much more overhead.

Much more overhead, really ?
So this is for performance reasons that all functionnal languages
promote singly-linked lists, while for instance in Linux every list
is implemented with a doubly linked list for purely ideological reasons ?

:-)