code: def joint_value_post_processor(a, _):
      
      # if there is only one value, return the value
      if len(a) == 1:
          return a[0]
  
      # if there are multiple values, calculate the joint value
      product = 1
      for v in a:
          new_value = (1-v)
          product = product * new_value
      joint_value = 1 - product
      return joint_value
docs: The final step in calculating joint values like disability weights.
      If the combiner is list_combiner then the effective formula is:
  
      .. math::
  
          value(args) = 1 -  \prod_{i=1}^{mutator count} 1-mutator_{i}(args)
  
      Parameters
      ----------
      a : List[pd.Series]
          a is a list of series, indexed on the population. Each series
          corresponds to a different value in the pipeline and each row
          in a series contains a value that applies to a specific simulant.