public static Atom getSharedAtom(RBond r1, RBond r2) {
		if (r1 == null || r2 == null || r1.equals(r2))
			throw new IllegalArgumentException("RBond error");
		if (r1.atom1 == r2.atom1 || r1.atom1 == r2.atom2)
			return r1.atom1;
		if (r1.atom2 == r2.atom1 || r1.atom2 == r2.atom2)
			return r1.atom2;
		return null;
	}