class-tree_index: alien:intUsed for very fast implementation of class->is_a
and object->instance_of.
After a modification on the class hierarchy (normally a class created),
PCE numbers the class-hierarchy top-down and left-to-right.
The slot -tree_index is assigned the current number when this class
is first reached, while the slot -neighbour_index is assigned the
current number after numbering the subtree below this class. Now,
a class A is_a B iff
(A<-tree_index) >= (B<-tree_index) AND
(A<-tree_index) < (B<-neighbour_index)
