A "contrib" example (contrib/src/libdu/smiles/du_part.c):

/*============================================================================
 *  du_part(ob) -- return part number of object `ob'
 */

int du_part(dt_Handle ob)
{
  int partno;
  dt_Handle parent;

  /*** If not calculated.  Note that partno will be zero if the property
    hasn't been set ever or if it is set to zero.  Either indicates that
    we must generate the parts.  ***/
  
  partno = dt_integer(ob, 4, "part");
  if (partno != 0) return (partno);
  
  parent = dt_parent(ob);
  if (fill_parts(parent) == 0)
    return (0);
  return (dt_integer(ob, 4, "part"));
}

/*============================================================================
 *  fill_parts(mol) -- fill part properties and return success
 */

static int fill_parts(dt_Handle mol)
{
  dt_Handle obs, ob;
  int       npart = 1;
  
  if ((TYP_MOLECULE != dt_type(mol)) &&
      (TYP_REACTION != dt_type(mol))) return 0; 
  
  /*** Zap the parts for the atoms and bonds ***/
  
  obs = dt_stream(mol, TYP_BOND);
  while (NULL_OB != (ob = dt_next(obs)))
    dt_setinteger(ob, 4, "part", 0);
  dt_dealloc(obs);
  
  obs = dt_stream(mol, TYP_ATOM);
  while (NULL_OB != (ob = dt_next(obs)))
    dt_setinteger(ob, 4, "part", 0);
  
  /*** Loop over atoms, setting parts with a depth-first-search. ***/
  
  dt_reset(obs);
  while (NULL_OB != (ob = dt_next(obs)))
    if (dt_integer(ob, 4, "part") == 0)
      set_part_dfs(ob, npart++);
  dt_dealloc(obs);
  
  dt_setinteger(mol, 9, "partcount", npart - 1);
  return (1);
}

/*============================================================================
 *  set_part_dfs(atom) -- set part of atom, and connected atoms, to npart
 */

static void set_part_dfs(dt_Handle atom, int npart)
{
  dt_Handle bond, bonds, toatom;
  
  dt_setinteger(atom, 4, "part", npart);
  
  bonds = dt_stream(atom, TYP_BOND);
  while (NULL_OB != (bond = dt_next(bonds)))
    {
      dt_setinteger(bond, 4, "part", npart);
      toatom = dt_xatom(atom, bond);
      if (dt_integer(toatom, 4, "part") == 0)
	set_part_dfs(toatom, npart);
    }
  dt_dealloc(bonds);
  return;
}