code: def prefix(expof10):
      '''
      Args:
  
          expof10 : Exponent of a power of 10 associated with a SI unit
              character.
  
      Returns:
  
          str : One of the characters in "yzafpnum kMGTPEZY".
      '''
      prefix_levels = (len(SI_PREFIX_UNITS) - 1) // 2
      si_level = expof10 // 3
  
      if abs(si_level) > prefix_levels:
          raise ValueError("Exponent out range of available prefixes.")
      return SI_PREFIX_UNITS[si_level + prefix_levelsf prefix(expof10):
      '''
      Args:
  
          expof10 : Exponent of a power of 10 associated with a SI unit
              character.
  
      Returns:
  
          str : One of the characters in "yzafpnum kMGTPEZY".
      '''
      prefix_levels = (len(SI_PREFIX_UNITS) - 1) // 2
      si_level = expof10 // 3
  
      if abs(si_level) > prefix_levels:
          raise ValueError("Exponent out range of available prefixes.")
      return SI_PREFIX_UNITS[si_level + prefix_levels]
docs: Args:
  
          expof10 : Exponent of a power of 10 associated with a SI unit
              character.
  
      Returns:
  
          str : One of the characters in "yzafpnum kMGTPEZY".