def next_reg_use( type , extra = {} )
      int = @symbol[1,3].to_i
      raise "No more registers #{self}" if int > 11
      sym = "r#{int + 1}".to_sym
      RegisterValue.new( sym , type, extra)
    end
def next_reg_use( type , extra = {} )
      int = @symbol[1,3].to_i
      raise "No more registers #{self}" if int > 11
      sym = "r#{int + 1}".to_sym
      RegisterValue.new( sym , type, extra)
    end
helper method to calculate with register symbols