code: def explicify_hydrogens(self):
          
          tmp = []
          for n, atom in self.atoms():
              if atom.element != 'H':
                  for _ in range(atom.get_implicit_h([x.order for x in self._adj[n].values()])):
                      tmp.append(n)
          for n in tmp:
              self.add_bond(n, self.add_atom(H), Bond())
  
          self.flush_cache()
          return len(tmp)
docs: add explicit hydrogens to atoms
  
          :return: number of added atoms