I'm just wondering. Let's say for example you are stuck on a cliff edge. And there just happens to be a handy tree. You've also got some rope to hand...funny how things like that work out . Anyway my question is. After using the tree and rope etc to rappel down is there any way of retrieving the rope? Like a special knot that keeps tight under friction but when the load is removed (say when you get to the bottom) you can loosen it and get the rope back from the bottom?