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