code: def get_doc_from_docid(self, docid, doc_type_name=None, inst=True):
          
          assert(docid is not None)
          if docid in self._docs_by_id:
              return self._docs_by_id[docid]
          if not inst:
              return None
          doc = self.__inst_doc(docid, doc_type_name)
          if doc is None:
              return None
          self._docs_by_id[docid] = doc
          return doc
docs: Try to find a document based on its document id. if inst=True, if it
          hasn't been instantiated yet, it will be.