class Fragment implements Iterable<Atom> {

	Atom getAtomByLocant(String locant) {
		Atom a =atomMapFromLocant.get(locant);
		if (a != null){
			return a;
		}
		Matcher m =MATCH_AMINOACID_STYLE_LOCANT.matcher(locant);
		if (m.matches()){//e.g. N5
			Atom backboneAtom =atomMapFromLocant.get(m.group(3));//the atom corresponding to the numeric or greek component
			if (backboneAtom==null){
				return null;
			}
			a = FragmentTools.getAtomByAminoAcidStyleLocant(backboneAtom,  m.group(1), m.group(2));
			if (a != null){
				return a;
			}
		}
		return null;
	}

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

}