code: @Override
  	public List<C> getCompoundsAt(int alignmentIndex) {
  		List<C> column = new ArrayList<C>();
  		for (S s : sequences) {
  			column.add(s.getCompoundAt(alignmentIndex));
  		}
  		return Collections.unmodifiableList(column);
  	}
docs: Get a list of compounds at a sequence position
  @param alignmentIndex
  @return compounds