Given a connected, undirected, simple graph G = (V,E) and p ≤ |V | pebbles labeled 1, . . . , p, a configuration of these p pebbles is an injective map assigning the pebbles to vertices of G. Let S and D be two such configurations. From a configuration, pebbles can move on G as follows: In each step, at most one pebble may move from the vertex it currently… (More)