code: def reaction_elements(reaction):
      
      c_elements = [coeff * met.elements.get('C', 0)
                    for met, coeff in iteritems(reaction.metabolites)]
      return [elem for elem in c_elements if elem != 0]
docs: Split metabolites into the atoms times their stoichiometric coefficients.
  
      Parameters
      ----------
      reaction : Reaction
          The metabolic reaction whose components are desired.
  
      Returns
      -------
      list
          Each of the reaction's metabolites' desired carbon elements (if any)
          times that metabolite's stoichiometric coefficient.