docs: def _reaction_to_dicts(reaction):
      """Convert a reaction to reduced left, right dictionaries."""
  
      def dict_from_iter_sum(it):
          d = {}
          for k, v in it:
              if k not in d:
                  d[k] = 0
              d[k] += v
          return d
  
      left = dict_from_iter_sum(reaction.left)
      right = dict_from_iter_sum(reaction.right)
  
      return left, right