old_contents: class OpenTracking(object):
      """
      Allows you to track whether the email was opened or not, by including a
      single pixel image in the body of the content. When the pixel is loaded,
      we log that the email was opened.
      """
  
      def __init__(self, enable=None, substitution_tag=None):
          """Create an OpenTracking to track when your email is opened.
  
          :param enable: If open tracking is enabled.
          :type enable: boolean, optional
          :param substitution_tag: Tag in body to be replaced by tracking pixel.
          :type substitution_tag: string, optional
          """
          self._enable = None
          self._substitution_tag = None
  
          if enable is not None:
              self.enable = enable
  
          if substitution_tag is not None:
              self.substitution_tag = substitution_tag
  
      @property
      def enable(self):
          """Indicates if this setting is enabled.
  
          :rtype: boolean
          """
          return self._enable
  
      @enable.setter
      def enable(self, value):
          """Indicates if this setting is enabled.
  
          :param value: Indicates if this setting is enabled.
          :type value: boolean
          """
          self._enable = value
  
      @property
      def substitution_tag(self):
          """Allows you to specify a substitution tag that you can insert in the
          body of your email at a location that you desire. This tag will be
          replaced by the open tracking pixel.
  
          :rtype: string
          """
          return self._substitution_tag
  
      @substitution_tag.setter
      def substitution_tag(self, value):
          """Allows you to specify a substitution tag that you can insert in the
          body of your email at a location that you desire. This tag will be
          replaced by the open tracking pixel.
  
          :param value: Allows you to specify a substitution tag that you can
                        insert in the body of your email at a location that you
                        desire. This tag will be replaced by the open tracking
                        pixel.
  
          :type value: string
          """
          self._substitution_tag = value
  
      def get(self):
          """
          Get a JSON-ready representation of this OpenTracking.
  
          :returns: This OpenTracking, ready for use in a request body.
          :rtype: dict
          """
          open_tracking = {}
          if self.enable is not None:
              open_tracking["enable"] = self.enable
  
          if self.substitution_tag is not None:
              open_tracking["substitution_tag"] = self.substitution_tag.get()
          return open_tracking
  
new_contents: class OpenTracking(object):
      """
      Allows you to track whether the email was opened or not, by including a
      single pixel image in the body of the content. When the pixel is loaded,
      we log that the email was opened.
      """
  
      def __init__(self, enable=None, substitution_tag=None):
          """Create an OpenTracking to track when your email is opened.
  
          :param enable: If open tracking is enabled.
          :type enable: boolean, optional
          :param substitution_tag: Tag in body to be replaced by tracking pixel.
          :type substitution_tag: OpenTrackingSubstitionTag, optional
          """
          self._enable = None
          self._substitution_tag = None
  
          if enable is not None:
              self.enable = enable
  
          if substitution_tag is not None:
              self.substitution_tag = substitution_tag
  
      @property
      def enable(self):
          """Indicates if this setting is enabled.
  
          :rtype: boolean
          """
          return self._enable
  
      @enable.setter
      def enable(self, value):
          """Indicates if this setting is enabled.
  
          :param value: Indicates if this setting is enabled.
          :type value: boolean
          """
          self._enable = value
  
      @property
      def substitution_tag(self):
          """Allows you to specify a substitution tag that you can insert in the
          body of your email at a location that you desire. This tag will be
          replaced by the open tracking pixel.
  
          :rtype: string
          """
          return self._substitution_tag
  
      @substitution_tag.setter
      def substitution_tag(self, value):
          """Allows you to specify a substitution tag that you can insert in the
          body of your email at a location that you desire. This tag will be
          replaced by the open tracking pixel.
  
          :param value: Allows you to specify a substitution tag that you can
                        insert in the body of your email at a location that you
                        desire. This tag will be replaced by the open tracking
                        pixel.
  
          :type value: string
          """
          self._substitution_tag = value
  
      def get(self):
          """
          Get a JSON-ready representation of this OpenTracking.
  
          :returns: This OpenTracking, ready for use in a request body.
          :rtype: dict
          """
          open_tracking = {}
          if self.enable is not None:
              open_tracking["enable"] = self.enable
  
          if self.substitution_tag is not None:
              open_tracking["substitution_tag"] = self.substitution_tag.get()
          return open_tracking
  
subject: Update substitution_tag requirements