code: def get_value_prob(self, attr_name, value):
          
          if attr_name not in self._attr_value_count_totals:
              return
          n = self._attr_value_counts[attr_name][value]
          d = self._attr_value_count_totals[attr_name]
          return n/float(d)
docs: Returns the value probability of the given attribute at this node.