code: @Override
      public void backTrack() {
          if (queryPath.isEmpty() || isGoal()) {
              map.clear();
              return;
          }
          if (isHeadMapped()) {
              return;
          }
          map.clear();
          for (int i = 0; i < queryPath.size() - 1; i++) {
              map.put(queryPath.get(i), targetPath.get(i));
          }
      }
docs: {@inheritDoc}