code: public IntronSequence addIntron(AccessionID accession, int begin, int end) throws Exception {
  		if (intronSequenceHashMap.containsKey(accession.getID())) {
  			throw new Exception("Duplicate accesion id " + accession.getID());
  		}
  		intronAdded = true;
  		IntronSequence intronSequence = new IntronSequence(this, begin, end); // working off the assumption that intron frame is always 0 or doesn't matter and same sense as parent
  		intronSequence.setAccession(accession);
  		intronSequenceList.add(intronSequence);
  		intronSequenceHashMap.put(accession.getID(), intronSequence);
  		return intronSequence;
  	}
docs: Add an Intron Currently used to mark an IntronSequence as a feature
  @param accession
  @param begin
  @param end
  @return intron sequence