def -(oth)
      if oth.is_a? Integer
        self.prefix - oth
      else
        (self.prefix - oth.prefix).abs
      end
    end
def -(oth)
      if oth.is_a? Integer
        self.prefix - oth
      else
        (self.prefix - oth.prefix).abs
      end
    end
Returns the difference between two
 prefixes, or a prefix and a number,
 as a Integer