docs: def serial_ppmap(func, fixed_arg, var_arg_iter):
      """A serial implementation of the "partially-pickling map" function returned
      by the :meth:`ParallelHelper.get_ppmap` interface. Its arguments are:
  
      *func*
        A callable taking three arguments and returning a Pickle-able value.
      *fixed_arg*
        Any value, even one that is not pickle-able.
      *var_arg_iter*
        An iterable that generates Pickle-able values.
  
      The functionality is::
  
          def serial_ppmap(func, fixed_arg, var_arg_iter):
              return [func(i, fixed_arg, x) for i, x in enumerate(var_arg_iter)]
  
      Therefore the arguments to your ``func`` function, which actually does the
      interesting computations, are:
  
      *index*
        The 0-based index number of the item being processed; often this can
        be ignored.
      *fixed_arg*
        The same *fixed_arg* that was passed to ``ppmap``.
      *var_arg*
        The *index*'th item in the *var_arg_iter* iterable passed to
        ``ppmap``.
  
      """
      return [func(i, fixed_arg, x) for i, x in enumerate(var_arg_iter)]