def plus(other)
      if other.is_a? Number
        operate(other, :+)
      elsif other.is_a?(Color)
        other.plus(self)
      else
        super
      end
    end
def plus(other)
      if other.is_a? Number
        operate(other, :+)
      elsif other.is_a?(Color)
        other.plus(self)
      else
        super
      end
    end
@param value [Numeric] The value of the number
 @param numerator_units [::String, Array<::String>] See \{#numerator\_units}
 @param denominator_units [::String, Array<::String>] See \{#denominator\_units}
 The SassScript `+` operation.
 Its functionality depends on the type of its argument:

 {Number}
 : Adds the two numbers together, converting units if possible.

 {Color}
 : Adds this number to each of the RGB color channels.

 {Value}
 : See {Value::Base#plus}.

 @param other [Value] The right-hand side of the operator
 @return [Value] The result of the operation
 @raise [Sass::UnitConversionError] if `other` is a number with incompatible units