code: public static String getNaturalAnalogueSequence(PolymerNotation polymer) throws HELM2HandledException, PeptideUtilsException, ChemistryException {
      checkPeptidePolymer(polymer);
      return FastaFormat.generateFastaFromPeptide(MethodsMonomerUtils.getListOfHandledMonomers(polymer.getListMonomers()));
    }
docs: method to produce for a peptide PolymerNotation the natural analogue
  sequence
  
  @param polymer PolymerNotation
  @return natural analogue sequence
  @throws HELM2HandledException if the polymer contains HELM2 features
  @throws PeptideUtilsException if the polymer is not a peptide
  @throws ChemistryException if the Chemistry Engine is not initialized