from the greatest element of 'rbtree' lower of equal than 'data'
to the lowest element of the tree 'rbtree' greater or equal than 'data'
It is more logical and natural:
if you have a tree of elements between [2, 4], and you want
to iterate all elements from 0 to 10, the old definition doesn't allow
you to iterate easily over this subset.
Patrick Pelissier
85a363d325