code: def add_annotation(self, about, content, motivated_by="oa:describing"):
          # type: (str, List[str], str) -> str
          
          self.self_check()
          curr = self.base_uri + METADATA + "/"
          content = [c.replace(curr, "").replace(self.base_uri, "../")
                     for c in content]
          uri = uuid.uuid4().urn
          ann = {
              "uri": uri,
              "about": about,
              "content": content,
              "oa:motivatedBy": {"@id": motivated_by}
          }
          self.annotations.append(ann)
          return uri
docs: Cheap URI relativize for current directory and /.