code: function addAttachAndSeqStore(db) {
      var attAndSeqStore = db.createObjectStore(ATTACH_AND_SEQ_STORE,
        {autoIncrement: true});
      attAndSeqStore.createIndex('seq', 'seq');
      attAndSeqStore.createIndex('digestSeq', 'digestSeq', {unique: true});
    }
docs: migration to version 4 (part 1)