code: def _generate_limit_items(lower, upper):
      
      # Use value + 0 to convert any -0.0 to 0.0 which looks better.
      if lower is not None and upper is not None and lower == upper:
          yield 'fixed', upper + 0
      else:
          if lower is not None:
              yield 'lower', lower + 0
          if upper is not None:
              yield 'upper', upper + 0
docs: Yield key, value pairs for limits dictionary.
  
      Yield pairs of key, value where key is ``lower``, ``upper`` or ``fixed``.
      A key, value pair is emitted if the bounds are not None.