code: public Monomer getSugarMonomer(MonomerStore monomerStore) {
      String sugarSymbol = getSugarSymbol();
      if (sugarSymbol != null && !sugarSymbol.equalsIgnoreCase("")) {
        try {
          Map<String, Monomer> monomers = monomerStore.getMonomers(Monomer.NUCLIEC_ACID_POLYMER_TYPE);
          Monomer m = monomers.get(sugarSymbol);
          return m;
        } catch (Exception ex) {
          LOG.info("Unable to get sugar monomer for "
              + sugarSymbol);
          return null;
        }
      } else {
        return null;
      }
    }
docs: get the sugar monomer, the return value could be null if the "nucleotide"
  does not has a sugar
  
  @param monomerStore - store in which sugar monomer is located
  @return sugar monomer