def _perform(environment)
      operator = "unary_#{@operator}"
      value = @operand.perform(environment)
      value.send(operator)
    rescue NoMethodError => e
      raise e unless e.name.to_s == operator.to_s
      raise Sass::SyntaxError.new("Undefined unary operation: \"#{@operator} #{value}\".")
    end
def _perform(environment)
      operator = "unary_#{@operator}"
      value = @operand.perform(environment)
      value.send(operator)
    rescue NoMethodError => e
      raise e unless e.name.to_s == operator.to_s
      raise Sass::SyntaxError.new("Undefined unary operation: \"#{@operator} #{value}\".")
    end
Evaluates the operation.

 @param environment [Sass::Environment] The environment in which to evaluate the SassScript
 @return [Sass::Script::Value] The SassScript object that is the value of the operation
 @raise [Sass::SyntaxError] if the operation is undefined for the operand