code: public String getLinkerNotation() {
      String pSymbol = getPhosphateSymbol();
      String result = null;
      if (null == pSymbol || pSymbol.length() == 0) {
        result = "";
      } else {
        if (pSymbol.length() > 1)
          result = "[" + pSymbol + "]";
        else
          result = pSymbol;
      }
      return result;
    }
docs: This method returns the HELM notation for nucleotide linker
  
  @return linker notation