def update(self, other):
        """ Merge two Textgroup Objects.

        - Original (left Object) keeps his parent.
        - Added document merges with work if it already exists

        :param other: Textgroup object
        :type other: CtsTextgroupMetadata
        :return: Textgroup Object
        :rtype: CtsTextgroupMetadata
        """
        if not isinstance(other, CtsTextgroupMetadata):
            raise TypeError("Cannot add %s to CtsTextgroupMetadata" % type(other))
        elif str(self.urn) != str(other.urn):
            raise InvalidURN("Cannot add CtsTextgroupMetadata %s to CtsTextgroupMetadata %s " % (self.urn, other.urn))

        for urn, work in other.works.items():
            if urn in self.works:
                self.works[urn].update(deepcopy(work))
            else:
                self.works[urn] = deepcopy(work)
            self.works[urn].parent = self
            self.works[urn].resource = None

        return self