code: def mass_fractions(self):
          r'''Dictionary of mass fractions for each atom in the mixture.
  
          Examples
          --------
          >>> Mixture(['CO2', 'O2'], zs=[0.5, 0.5]).mass_fractions
          {'C': 0.15801826905745822, 'O': 0.8419817309425419}
          '''
          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
          return mass_fractions(thingsf mass_fractions(self):
          r'''Dictionary of mass fractions for each atom in the mixture.
  
          Examples
          --------
          >>> Mixture(['CO2', 'O2'], zs=[0.5, 0.5]).mass_fractions
          {'C': 0.15801826905745822, 'O': 0.8419817309425419}
          '''
          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
          return mass_fractions(things)
docs: r'''Dictionary of mass fractions for each atom in the mixture.
  
          Examples
          --------
          >>> Mixture(['CO2', 'O2'], zs=[0.5, 0.5]).mass_fractions
          {'C': 0.15801826905745822, 'O': 0.8419817309425419}