code: final SMARTSAtomInvariants invariants(final IAtom atom) {
          final SMARTSAtomInvariants inv = atom.getProperty(SMARTSAtomInvariants.KEY);
          if (inv == null)
              throw new NullPointerException(
                      "Missing SMARTSAtomInvariants - please compute these values before matching.");
          return inv;
      }
docs: Access the atom invariants for this atom. If the invariants have not been
  set an exception is thrown.
  
  @param atom the atom to obtain the invariants of
  @return the atom invariants for the atom
  @throws NullPointerException thrown if the invariants were not set