code: private static String encodeHalPath(int dist, IAtom a, IAtom b) {
          return dist + "_" + (isHalogen(a) ? "X" : a.getSymbol()) + "_" +
                 (isHalogen(b) ? "X" : b.getSymbol());
      }
docs: Encodes name for halogen paths
  @param dist
  @param a
  @param b
  @return