Before revision:   There are no known efficient algorithms to compute rotation distance between trees , where rotations are permitted at any node . Limiting the allowed locations of where rotations are permitted gives rise to a number of notions of restricted rotation distances.
After revision:  The one-dimensional skeleta of associahedra are rotation graphs, where two vertices representing trees are connected by an edge if they differ by a single rotation. There are no known efficient algorithms to compute rotation distance between trees , where rotations are permitted at any node . Limiting the allowed locations of where rotations are permitted gives rise to a number of notions of restricted rotation distances.