code: def accessible(self,fromstate, tostate):
          
          if (not (fromstate in self.nodes)) or (not (tostate in self.nodes)) or not (fromstate in self.edges_out):
              return 0
          if tostate in self.edges_out[fromstate]:
              return self.edges_out[fromstate][tostate]
          else:
              return 0
docs: Is state tonode directly accessible (in one step) from state fromnode? (i.e. is there an edge between the nodes). If so, return the probability, else zero