code: def get_normalized_quantity_modifier(self, quantity):
          
          modifier = self._quantity_modifiers.get(quantity, self._default_quantity_modifier)
          if modifier is None:
              return (trivial_callable, quantity)
  
          if callable(modifier):
              return (modifier, quantity)
  
          if isinstance(modifier, (tuple, list)) and len(modifier) > 1 and callable(modifier[0]):
              return modifier
  
          return (trivial_callable, modifier)
docs: Retrive a quantify modifier, normalized.
          This function would also return a tuple, with the first item a callable,
          and the rest native quantity names
  
          Parameters
          ----------
          quantity : str
              name of the derived quantity to get
  
          Returns
          -------
          tuple : (callable, quantity1, quantity2...)