<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def decipher_atom_keys(self, forcefield='DLF', dict_key='atom_ids'):
 """ Decipher force field atom ids. This takes all values in :attr:`MolecularSystem.system['atom_ids']` that match force field type criteria and creates :attr:`MolecularSystem.system['elements']` with the corresponding periodic table of elements equivalents. If a forcefield is not supported by this method, the :func:`MolecularSystem.swap_atom_keys()` can be used instead. DLF stands for DL_F notation. See: C. W. Yong, Descriptions and Implementations of DL_F Notation: A Natural Chemical Expression System of Atom Types for Molecular Simulations, J. Chem. Inf. Model., 2016, 56, 1405–1409. Parameters forcefield : :class:`str` The forcefield used to decipher atom ids. Allowed (not case sensitive):
 'OPLS', 'OPLS2005', 'OPLSAA', 'OPLS3', 'DLF', 'DL_F'. (default='DLF') dict_key : :class:`str` The :attr:`MolecularSystem.system` dictionary key to the array containing the force field atom ids. (default='atom_ids') Returns ------- None : :class:`NoneType` """

        # In case there is no 'atom_ids' key we try 'elements'. This is for
        # XYZ and MOL files mostly. But, we keep the dict_key keyword for
        # someone who would want to decipher 'elements' even if 'atom_ids' key
        # is present in the system's dictionary.
        if 'atom_ids' not in self.system.keys():
            dict_key = 'elements'
        # I do it on temporary object so that it only finishes when successful
        temp = deepcopy(self.system[dict_key])
        for element in range(len(temp)):
            temp[element] = "{0}".format(
                decipher_atom_key(
                    temp[element], forcefield=forcefield))
        self.system['elements'] = temp