def opponent=(opponent)
      @opponent = case opponent
        when nil     then nil
        when Fixnum  then opponent
        when /^\s*$/ then nil
        else opponent.to_i
      end
      raise "invalid opponent number (#{opponent})" if @opponent == 0 && !opponent.to_s.match(/\d/)
      raise "opponent number and player number (#{@opponent}) must be different" if @opponent == player
      self.rateable = true if @opponent
    end
def opponent=(opponent)
      @opponent = case opponent
        when nil     then nil
        when Fixnum  then opponent
        when /^\s*$/ then nil
        else opponent.to_i
      end
      raise "invalid opponent number (#{opponent})" if @opponent == 0 && !opponent.to_s.match(/\d/)
      raise "opponent number and player number (#{@opponent}) must be different" if @opponent == player
      self.rateable = true if @opponent
    end
Opponent player number. Either absent (_nil_) or any integer except the player number.