code: function verifyAttachment(digest, callback) {
        txn.get(stores.attachmentStore, digest, function (levelErr) {
          if (levelErr) {
            var err = pouchdbErrors.createError(pouchdbErrors.MISSING_STUB,
                                  'unknown stub attachment with digest ' +
                                  digest);
            callback(err);
          } else {
            callback();
          }
        });
      }
docs: verify any stub attachments as a precondition test