code: public ExonSequence addExon(AccessionID accession, int begin, int end) throws Exception {
  		if (exonSequenceHashMap.containsKey(accession.getID())) {
  			throw new Exception("Duplicate accesion id " + accession.getID());
  		}
  
  		ExonSequence exonSequence = new ExonSequence(this, begin, end); //sense should be the same as parent
  		exonSequence.setAccession(accession);
  		exonSequenceList.add(exonSequence);
  		exonSequenceHashMap.put(accession.getID(), exonSequence);
  		return exonSequence;
  	}
docs: Add an ExonSequence mainly used to mark as a feature
  @param accession
  @param begin
  @param end
  @return exon sequence