code: public Monomer getPhosphateMonomer() {
      MonomerFactory factory = null;
      try {
        factory = MonomerFactory.getInstance();
      } catch (Exception ex) {
        Logger.getLogger(Nucleotide.class.getName()).log(Level.SEVERE, "Unable to initialize monomer factory", ex);
      }
      return getPhosphateMonomer(factory.getMonomerStore());
    }
docs: return the phosphate monomer of this nucleotide
  
  @return phosphate monomer