docs: def infer_si_prefix(f):
      """
      Infers the SI prefix of a value *f* and returns the string label and decimal magnitude in a
      2-tuple. Example:
  
      .. code-block:: python
  
          infer_si_prefix(1)     # -> ("", 0)
          infer_si_prefix(25)    # -> ("", 0)
          infer_si_prefix(4320)  # -> ("k", 3)
      """
      if f == 0:
          return "", 0
      else:
          mag = 3 * int(math.log10(abs(float(f))) // 3)
          return si_refixes[mag], mag