code: @Override
      public boolean matches(IAtom atom) {
          if (mass == 0)
              return atom.getMassNumber() == null;
          else
              return atom.getMassNumber() != null && mass == atom.getMassNumber();
      }
docs: /*
  (non-Javadoc)
  @see
  org.openscience.cdk.isomorphism.matchers.smarts.SMARTSAtom#matches(org
  .openscience.cdk.interfaces.IAtom)