docs: def charge_balance(model):
      """Calculate the overall charge for all reactions in the model.
  
      Yield (reaction, charge) pairs.
  
      Args:
          model: :class:`psamm.datasource.native.NativeModel`.
      """
  
      compound_charge = {}
      for compound in model.compounds:
          if compound.charge is not None:
              compound_charge[compound.id] = compound.charge
  
      for reaction in model.reactions:
          charge = reaction_charge(reaction.equation, compound_charge)
          yield reaction, charge