Gets the lower bound node corresponding to the given key,
or %NULL if the tree is empty or all the nodes in the tree
have keys that are strictly lower than the searched key.
The lower bound is the first node that has its key greater
than or equal to the searched key.
Gets the lower bound node corresponding to the given key, or %NULL if the tree is empty or all the nodes in the tree have keys that are strictly lower than the searched key.
The lower bound is the first node that has its key greater than or equal to the searched key.