One of the central questions in space complexity is whether nondeterministic logspace computations (NL) can be simulated in unambiguous logspace (UL). A stronger notion, reach unambiguity (RUL $\subseteq$ UL $\cap$ coUL), requires every configuration reachable from the start to have exactly one computation path (Buntrock et.al. 91). While directed graph ... more >>>