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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def atom_by_serialnumber(self):
 """Provides a dictionary mapping serial numbers to their atom objects."""

        atm_by_snum = {}
        for atom in self.model.atoms:
            atm_by_snum[atom.serialNumber] = atom
        return atm_by_snum