In addition to the tree itself, MULTOPS maintains a doubly-linked list of
pointers to nodes in the tree using prev and next in
Table. Each time a new node is created in the tree, i.e., expansion
occurs, a pointer to that node is added at the end of the linked list. During
a cleanup, the list is traversed. A node (and all its children) is deleted
when the sum of all from-rates and the sum of all to-rates in that node are
both lower than the expand threshold. (Both sums are, by definition, stored as
from-rate and to-rate in the parent record of that node; hence the need for
the parent pointer in Table.) The root node is never deleted.
The list is either traversed backwards or forwards to avoid checking the same
nodes every time thereby causing starvation-like phenomena.
To avoid heavy memory fluctuations and to avoid spending too much time on a
single cleanup, contraction stops when a certain fraction 
of all
allocated memory has been freed. If none of the nodes can be deleted, but
memory is at its imposed maximum, then some nodes must be deleted. In
that case, the expand threshold is decreased by some factor and the cleanup
starts again. This may have to be repeated multiple times until fraction
of all memory has been freed.