I have an 18 year old lilac tree (see attached), that is in need of a radical pruning, without killing the tree. It is 9' tall, the top portion is 6.5' wide. The trunk is 4" in diameter. I've read somewhere on the internet that I can over the course of several pruning over several years, I can cut back a 1/3 of the length of the branches each year?? I need some good instructions on how to safely decrease the size of the top portion of this tree, without killing it. I realize it won't be flowering while I'm doing this radical pruning, which I'm fine with. Ultimately I'd like to get the top back to a respectable 3' wide or so. Any detailed instructions are greatly appreciated!