docs: def _get_original_composition_ratio(self, reaction):
          """
          Returns the molar mixing ratio between the reactants with ORIGINAL (
          instead of processed) compositions for a reaction.
  
          Args:
              reaction (Reaction): Reaction object that contains the original
                  reactant compositions.
  
          Returns:
              The molar mixing ratio between the original reactant
              compositions for a reaction.
          """
          if self.c1_original == self.c2_original:
              return 1
          c1_coeff = reaction.get_coeff(self.c1_original) \
              if self.c1_original in reaction.reactants else 0
          c2_coeff = reaction.get_coeff(self.c2_original) \
              if self.c2_original in reaction.reactants else 0
          return c1_coeff * 1.0 / (c1_coeff + c2_coeff)