  In the problem of multi-robot motion planning, a group of robots, placed in a
polygonal domain with obstacles, must be moved from their starting positions to
a set of target positions. We consider the specific case of unlabeled disc
robots of two different sizes. That is, within one class of robots, where a
class is given by the robots' size, any robot can be moved to any of the
corresponding target positions. We prove that the decision problem of whether
there exists a schedule moving the robots to the target positions is
PSPACE-hard.
