docs: def get_default_compartment(model):
      """Return what the default compartment should be set to.
  
      If some compounds have no compartment, unique compartment
      name is returned to avoid collisions.
      """
      default_compartment = 'c'
      default_key = set()
      for reaction in model.reactions:
          equation = reaction.equation
          if equation is None:
              continue
  
          for compound, _ in equation.compounds:
              default_key.add(compound.compartment)
  
      if None in default_key and default_compartment in default_key:
          suffix = 1
          while True:
              new_key = '{}_{}'.format(default_compartment, suffix)
              if new_key not in default_key:
                  default_compartment = new_key
                  break
              suffix += 1
  
      if None in default_key:
          logger.warning(
              'Compound(s) found without compartment, default'
              ' compartment is set to {}.'.format(default_compartment))
      return default_compartment