code: private static int implH(IAtom atom) {
          Integer h = atom.getImplicitHydrogenCount();
          if (h != null) return h;
          if (atom instanceof IPseudoAtom) return 0;
          throw new NullPointerException("a non-pseudoatom had unset hydrogen count");
      }
docs: Access implicit hydrogen count of the atom defaulting to 0 for pseudo
  atoms.
  
  @param atom an atom
  @return the implicit hydrogen count
  @throws NullPointerException the atom was non-pseudo at did not have an
  implicit hydrogen count