code: public Monomer getPhosphateMonomer(MonomerStore monomerStore) {
      String phosphateSymbol = getPhosphateSymbol();
      if (phosphateSymbol != null && !phosphateSymbol.equalsIgnoreCase("")) {
        try {
          Map<String, Monomer> monomers = monomerStore.getMonomers(Monomer.NUCLIEC_ACID_POLYMER_TYPE);
          Monomer m = monomers.get(phosphateSymbol);
          return m;
        } catch (Exception ex) {
          LOG.info("Unable to get phosphate monomer for "
              + phosphateSymbol);
          return null;
        }
      } else {
        return null;
      }
    }
docs: return the phosphate monomer of this nucleotide
  
  @param monomerStore - store in which phosphate monomer is located
  @return phosphate monomer