code: private static void incrementImplHydrogenCount(final IAtom atom) {
          Integer hCount = atom.getImplicitHydrogenCount();
  
          if (hCount == null) {
              if (!(atom instanceof IPseudoAtom))
                  throw new IllegalArgumentException("a non-pseudo atom had an unset hydrogen count");
              hCount = 0;
          }
  
          atom.setImplicitHydrogenCount(hCount + 1);
      }
docs: Increment the implicit hydrogen count of the provided atom. If the atom
  was a non-pseudo atom and had an unset hydrogen count an exception is
  thrown.
  
  @param atom an atom to increment the hydrogen count of