void parent(final TreeNode<T> parent) {
		_parent = parent;
	}
Sets this node's parent, but does not change the parent's child array.
This method is called from {@code insert()} and {@code remove()} to
reassign a child's parent, and it should not be messaged from anywhere
else.

@param parent this node's new parent
jenetics.ext/src/main/java/io/jenetics/ext/util/TreeNode.java