docs: def reaction_charge(reaction, compound_charge):
      """Calculate the overall charge for the specified reaction.
  
      Args:
          reaction: :class:`psamm.reaction.Reaction`.
          compound_charge: a map from each compound to charge values.
      """
  
      charge_sum = 0.0
      for compound, value in reaction.compounds:
          charge = compound_charge.get(compound.name, float('nan'))
          charge_sum += charge * float(value)
      return charge_sum