*Add a tag this taskpaper object*

        **Key Arguments:**
            - ``tag`` -- the tag to add to the object

        **Usage:**

            .. code-block:: python 

                aTask.add_tag("@due")

def add_tag(
            self,
            tag):
        """*Add a tag this taskpaper object*

        **Key Arguments:**
            - ``tag`` -- the tag to add to the object

        **Usage:**

            .. code-block:: python 

                aTask.add_tag("@due")
        """

        if tag.replace("@", "") in self.tags:
            return

        self.refresh
        oldContent = self.to_string(indentLevel=1)
        self.tags += [tag.replace("@", "")]
        newContent = self.to_string(indentLevel=1)

        # ADD DIRECTLY TO CONTENT IF THE PROJECT IS BEING ADDED SPECIFICALLY TO
        # THIS OBJECT
        self.parent._update_document_tree(
            oldContent=oldContent,
            newContent=newContent
        )

        self.refresh
        return None