docs: def detect_extracellular_compartment(model):
      """Detect the identifier for equations with extracellular compartments.
  
      Args:
          model: :class:`NativeModel`.
      """
      extracellular_key = Counter()
  
      for reaction in model.reactions:
          equation = reaction.equation
          if equation is None:
              continue
  
          if len(equation.compounds) == 1:
              compound, _ = equation.compounds[0]
              compartment = compound.compartment
              extracellular_key[compartment] += 1
      if len(extracellular_key) == 0:
          return None
      else:
          best_key, _ = extracellular_key.most_common(1)[0]
  
      logger.info('{} is extracellular compartment'.format(best_key))
  
      return best_key