class Fragment implements Iterable<Atom> {

	Bond findBond(int ID1, int ID2) {
		Atom a = atomMapFromId.get(ID1);
		if (a != null){
			for (Bond b : a.getBonds()) {
				if((b.getFrom() == ID1 && b.getTo() == ID2) ||
						(b.getTo() == ID1 && b.getFrom() == ID2)) {
					return b;
				}
			}
		}
		return null;
	}

	  Fragment(Element tokenEl);
	  Fragment(String type);

}