code: def atom_fractions(self):
          r'''Dictionary of atomic fractions for each atom in the mixture.
  
          Examples
          --------
          >>> Mixture(['CO2', 'O2'], zs=[0.5, 0.5]).atom_fractions
          {'C': 0.2, 'O': 0.8}
          '''
          things = dict()
          for zi, atoms in zip(self.zs, self.atomss):
              for atom, count in atoms.iteritems():
                  if atom in things:
                      things[atom] += zi*count
                  else:
                      things[atom] = zi*count
  
          tot = sum(things.values())
          return {atom : value/tot for atom, value in things.iteritems()f atom_fractions(self):
          r'''Dictionary of atomic fractions for each atom in the mixture.
  
          Examples
          --------
          >>> Mixture(['CO2', 'O2'], zs=[0.5, 0.5]).atom_fractions
          {'C': 0.2, 'O': 0.8}
          '''
          things = dict()
          for zi, atoms in zip(self.zs, self.atomss):
              for atom, count in atoms.iteritems():
                  if atom in things:
                      things[atom] += zi*count
                  else:
                      things[atom] = zi*count
  
          tot = sum(things.values())
          return {atom : value/tot for atom, value in things.iteritems()}
docs: r'''Dictionary of atomic fractions for each atom in the mixture.
  
          Examples
          --------
          >>> Mixture(['CO2', 'O2'], zs=[0.5, 0.5]).atom_fractions
          {'C': 0.2, 'O': 0.8}