code: def normalize_bound(bound):
      
      min_, max_ = bound
  
      if min_ is None:
          min_ = -float('inf')
  
      if max_ is None:
          max_ = float('inf')
  
      return min_, max_
docs: Replace ``None`` with + or - inf in bound tuples.
  
      Examples
      --------
      >>> normalize_bound((2.6, 7.2))
      (2.6, 7.2)
  
      >>> normalize_bound((None, 7.2))
      (-inf, 7.2)
  
      >>> normalize_bound((2.6, None))
      (2.6, inf)
  
      >>> normalize_bound((None, None))
      (-inf, inf)
  
      This operation is idempotent:
  
      >>> normalize_bound((-float("inf"), float("inf")))
      (-inf, inf)