<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def explicify_hydrogens(self):
 """ add explicit hydrogens to atoms :return: number of added atoms """

        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)